Has anyone ever tried using one of these cheapo ebay top cases?  I know it isn't weatherproof or near the quality of a Givi, but I could use the extra space.  I very rarely have a passenger, but when I do they like to have a backrest of some sort.

http://www.ebay.com/itm/262331814363?_trksid=p2060353.m1438.l2649&ssPageName=STRK%3AMEBIDX%3AIT

I've got a tank bag bigger than that. 

I have one almost identical to that.  I found that the backrest ended up a little too far back to be of much practical use.  I suppose a passenger could lean back on it occasionally for a bit of change of position. Anyway, I've had mine on the bike for maybe 10 years now.  The latch is starting to get a little bit sticky.  I'm leaning toward getting another one like it.  It is cheaper in construction of course at that price point.  I have a Givi on my other bike. I'd say I'm pretty equally pleased with both boxes considering the price points.  If it's made like mine, it's basically relatively thin fiberglass.  I added an LED strip under the reflector wired to the brake light.  One thing I do like about my cheapo box over the Givi is that you don't have to lock the cheap box to latch it.  With the Givi it is locked all the time the key is out.  I'm modifying that as we speak actually.  But that's another matter.

Welcome to the site, Holmes. 
Let's hear about this bike of yours...

I bought the case and added it today.  Fabbed up a mount using angle aluminum bolted together to make a channel and mounted to the factory rack holes.  It's a little small, but I can fit a full-faced helmet in it.  For $35 and some aluminum I think it's pretty good.

I bought the case and added it today.  Fabbed up a mount using angle aluminum bolted together to make a channel and mounted to the factory rack holes.  It's a little small, but I can fit a full-faced helmet in it.  For $35 and some aluminum I think it's pretty good.

I think so too. If it does what you need and it looks good.