After investigating various options as a top box and what is available in SA , I decided to fit a Shad SH 48 with the optional backrest.
In order to fit the box as adapter plate is necessary which bolts onto the existing rack using the existing 4 holes but with additional spacers and longer bolts - all supplied.
When I first tired to loosen the allen head bolts holding the OE rack down , the allen key started to turn inside the heads. I sprayed some Q 20 on the threads , adjourned for a tea break and they then removed quite easily.
All holes lined up well on the mounting plate , the only fiddly bit was attaching the box base plate to the adapter plate as there is not much clearance for the washer and nyloc nuts. This I managed using a magnetic screwdriver on the head , holding the the washer with long nose pliers and putting in position where the magnet held it in place. For the nuts I used long nose Vice-grips and with a bit of manoeuvring manage to fasten all together.
The optional backrest requires attaching to the box by drilling 4 holes and fastening.
Once attached the box seems very firm with no real movement but haven't tried on the road as yet.
The box comes with a suitcase type handle for easy carrying and is easy to take off and on.
Here are some pics of the set up.

Good post/photos, It's interesting to see that Shad has bolstered their mounting plate so it doesn't depend on the stock luggage rack for anything.

Mine didn't even come with that "u" shaped portion.  I called and asked them to send it to me and they sent me another of the plastic plates.  I gave up and just bolted it down to Phil's rack.  I haven't had any issues--even after a 1300 mile trip.

The U shaped metal adapter bracket is sold separately under code KOGT 18 St.

Ahhh, that explains it.  I think I'm good with what I got.  Thanks though.

I have the Shad 50L in Candy red bolted to the OEM rack.  Took it to Alaska over 9K miles, fully loaded and have had no problem.

Your post is the first I've seen of the optional Shad metal mounting plate and it got me wondering if one could attach a scooter backrest, similar to the two pictured below, directly to the leading edge of the plate in a way that would'nt interfere with the trunk? Some people want both a trunk and backrest at the same time. Does it look like this could be done in a safe sturdy installation? I'm talking about attaching the mounting tabs of the backrest to the Shad metal plate without the plate shown in these photos.

Hello Phil,
In answer to your post here are some thoughts.
An optional backrest is available for the top box which I initially fitted . See pic below. However when my wife tried it out , she found that it restricted her space and was not very comfortable so I removed it and she is now happy . These backrests are about 75 mm at the thickest part on the top pad and about 50 mm at the thickest part of the bottom pad. She is quite slim at 5' 4 and 135 lbs. If the backrest was essential, one could rotate the plate and redrill to fit the bike rack and box mounting plate which would move the whole system back.
On the question of fitting an additional backrest , here are some of the dimensions.
The plate is 3 mm thick and the flat part is about 160 mm wide.
The plate extends about 20 mm in front of the bike rack.
The box extends about 75 mm from the front of the bike rack and about 55 mm in front of the mounting plate.
The clearance between the mounting plate and the bike rack is about 13 mm. See pic.
The scooter backrest would need to be mounted between the plate and bike back rest in the 13 mm gap.
Holes could be drilled through slots in the box plastic plate and through the metal mounting plate . These would be about 14 mm  from the front of the metal mounting plate and about 140 mm apart . In the pic these slots are just below the top red fastener. The curve of the metal plate underneath can be seen.
I think the biggest problem would be lack of space for the passenger unless the whole system was moved back