I'm thinking of getting a bag to strap on to my pillion seat, as I'm not riding with a passenger lately on long trips.

What are you using, if anything? Pics?

Well I want my bag waterproof and able to hold a week's worth of stuff without using the side cases cause I have other stuff there. I have the Wolfman dry bag in medium size. It just covers the inside halves of the side cases.

I use a Motocentric Mototrek roll tail bag, Comes with bungess built in to quickly attach to your bike. It fits perfectly on the rear seat. I fit a weeks worth of cloths in it. 

http://www.motorcycle-superstore.com/28901/i/motocentric-mototrek-roll-tail-bag

I used to use a "T bag" style motorcycle specific luggage but I had to put a rain fly over it when in the rain. So I got a simple dry bag from an outdoor store (don't remember which one) and it is water proof without a stupid rain fly. I just strap it on with a couple of bungees and off I go. Looks a lot like this:

I have a 40 70 liter drysack that I use on road trips.  When it's sufficiently cool out, we wear vented leather top and bottom. If its cold, I might have nylon long underwear on or maybe even heated gear.  When it's too hot for leather, we switch to mesh but they are different sizes.  The drysack expands and contracts and I use RokStraps to manage the bigger/smaller size of it. Works great but it's a pain to get in the side bags.  I have a 46L Givi case but I hate having weight high and to the rear, plus the drysack works better with leathers, which are heavy.

Following what PiP said, unless you just need a gigantic bag, get one that is 20" long/wide or less. When you strap it across the seat, the ends won't cover the lock and latch mechanism of the side cases. The Wolfman small is this size; I wish I had gotten that size. You can always strap it long ways behind you, which avoids this issue altogether.

"What are you using for a pillion seat bag?"

Hey now! That's my wife you're talking about!         

That's pretty funny 'rad.

One thing I don't like about Rhino and PiP's bags is the lack of built-in compression straps (that I can see). While the Wolfman bag isn't perfect, this is one feature that makes for an easy, clean, and compact package on the pillion (NOOOOOOOO, I'm not describing YOUR pillion, Conrad!). Some people don't like roll-top dry bags, but I don't put anything in mine that I need while riding. Strictly for things I need at a destination. Road essentials go in side cases or tank bag. The design of the opening is very good; it's easy to pack the Wolfman dry bags

Things about the Wolfman that aren't perfect? The D-rings are plastic; I would prefer metal, though this may not matter in the long run. I've not heard of anyone breaking one. The bag only comes with two tie-down straps, which I thought was a cheap-out by Wolfman. They should have included four tie-down straps for the price you pay for this bag. The velcro handle on the carrying straps is a stupid design. When you gather the two handles and attach the velcro, you have this big, empty vinyl tube filling your hand that wants to be a handle. Maybe I'm doing it wrong, but the handle could be better. Finally, they did not supply keepers for the compression straps, so you have all these stray straps flapping around unless you tuck them every which way.  I made my own keepers out of Gorilla tape, which can be seen in the pic of the bag I posted.

Anyway, I'm not trying to make anyone buy or steer clear of the Wolfman bag; just posting some things to think about and look for when selecting a bag.

That's pretty funny 'rad.

One thing I don't like about Rhino and PiP's bags is the lack of built-in compression straps (that I can see). While the Wolfman bag isn't perfect, this is one feature that makes for an easy, clean, and compact package on the pillion (NOOOOOOOO, I'm not describing YOUR pillion, Conrad!). Some people don't like roll-top dry bags, but I don't put anything in mine that I need while riding. Strictly for things I need at a destination. Road essentials go in side cases or tank bag. The design of the opening is very good; it's easy to pack the Wolfman dry bags

Things about the Wolfman that aren't perfect? The D-rings are plastic; I would prefer metal, though this may not matter in the long run. I've not heard of anyone breaking one. The bag only comes with two tie-down straps, which I thought was a cheap-out by Wolfman. They should have included four tie-down straps for the price you pay for this bag. The velcro handle on the carrying straps is a stupid design. When you gather the two handles and attach the velcro, you have this big, empty vinyl tube filling your hand that wants to be a handle. Maybe I'm doing it wrong, but the handle could be better. Finally, they did not supply keepers for the compression straps, so you have all these stray straps flapping around unless you tuck them every which way.  I made my own keepers out of Gorilla tape, which can be seen in the pic of the bag I posted.

Anyway, I'm not trying to make anyone buy or steer clear of the Wolfman bag; just posting some things to think about and look for when selecting a bag.

Actually this does look pretty nice. About 3x what I paid for mine but I do get some water through the zipper in heavy rain where this roll top looks completely water proof.

http://www.revzilla.com/motorcycle/wolfman-expedition-dry-saddle-bags

That price is f'in insane.  My dry sack costs $40 on Amazon. 

Mounting it in line with the bike is a great idea, I will attempt that.  I have no clue why I have never attempted that?

I like that it is very expandable/contractable and I use RokStraps, which are very clever, they are basically variable length bungee cords.  Warning, mine are about 7/8" wide, a friend got the same bag because he liked mine but he bought very small/narrow RokStraps, which look entirely too wimpy to me.  The RokStraps are not intrinsic to the bag, but that does mean you can use them for other things, not just for the bag.

If it tells you anything about the solution, a fellow C14 owner took one look at my system and went out and bought same (except for buying the straps too small).

Straps
http://www.amazon.com/ROK-Straps-ROK-10025-Black-Motorcycle/dp/B00SXJLLBW?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_search_detailpage

Bag  Hmm, price went up, now $50, I got it for $40.
http://www.amazon.com/DP-D1GR-Waterproof-Duffel-Green-Large/dp/B002PKF4WM?ie=UTF8&psc=1&redirect=true&ref_=oh_aui_search_detailpage

Warning, that bag is HUGE, 70 liters.  If you need smaller, get smaller.

 

I'm thinking of getting a bag to strap on to my pillion seat, as I'm not riding with a passenger lately on long trips.

What are you using, if anything? Pics?

The bags shown so far will do the job and most are cheaper than what I have, but the bag I have seems made for the pylon so I thought I'd share it. After it was initially paid for (sticker shock) I'm very happy with it.
Here is the current version, mine is older and looks a bit different. You may find one like mine for sale used. It's not as slick looking as the new design but I do think the older one had a better carrying handle. It is expandable with with a zipper but I always manage to fill it up to full height. It comes with four straps that loop around the grab rails and then go through cam locks attached to the bag. There are elastic bands sewn to the bag for holding the extra strapping after it is cinched down tight. They also make small cable locks to secure the bag and most of it's contents. Of course it is minimal in security, but it protects against the grab and go type thief that might mess with it while the bike is parked at a busy gas/break stop and you are inside parked in front of a urinal.

@Rhino... I wanted the Givi 40L roll top bag, but I was in a hurry and amazon was out of stock.  It goes for $95 versus the more expensive Wolfman.  I've never seen a bad review on this Givi bag:

http://www.amazon.com/Givi-Weatherproof-WP400-Roll-Bag/dp/B006V1SYWS/ref=sr_1_1?s=automotive&ie=UTF8&qid=1464977924&sr=1-1&keywords=givi+40l

@PiP... it appears that the DryPak bag in your link DOES have built-in compression straps on the end and across the top, though it is still hard to determine from the amazon pics.  And it is a roll-top bag, which is better for water resistance. If that is the case, then the only thing against it for me would be the size; I personally would not need a bag larger than the 40L I currently own.

Here is something else to think about.  If you are a solo rider with a bag strapped to the pillion, then you might like something substantial there to act as a backrest.  Take a look at PiP's bag; since it is 70L and it's not packed full, it tends to just lay down relatively flat.  If you pack a smaller bag until it is full, or have the ability to pack and cinch a larger bag into a more compact shape, then you can strap it down such that it acts as a padded backrest.

Don't have one myself (yet?) but I've read good things about this Bagster Spider. Looks good, expandable and doubles as a backpack. I think they say it's French and takes a while to get to ya, but here's where to order: http://www.real-rider.com/bagster/?sort=featured&page=2

I think you can even select from a few colors for the side logo / graphic.

Edit: I found email correspondence from January when I was thinking about getting one. It includes the mounting attachments, is available in black, black / blue, black / yellow, black / green and as in the pic below black / red, and was $154.67 + $15.00 Shipping from Joe of bagster@CIMAINT.COM at 1585 Beverly Court # 118, Aurora, IL 60502 Ph: 630-701-1601 Toll-free: 866-243-5638

! No longer available

...
Here is something else to think about.  If you are a solo rider with a bag strapped to the pillion, then you might like something substantial there to act as a backrest.  ...

That...

MotoDry ZXR-1 REAR ROLL BAG - ECO SERIES. Has straps which go under the rack rails and secure bag to the bike. Plenty of room,

• Heavy duty reinforced 600 denier material (PU backed) – stylish modern design
• Internal “Easy Access” security pocket with see through design – see what’s in there
• Hard slip in internal base support to hold shape (if required – slip in or slip out)
• 3M Scotchlite reflectives for added night safety
• Robust carry handle and clip on shoulder strap
• Huge storage capacity 40 litres
• Two side pockets for extra “Easy Access” storage
• Extra clips and straps for mounting onto seat – or just use your own elastic straps to secure to seat top using the four end rings
• Storm/rain covers included
500mm L x 270mm W x 310mm H