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.