Hi Jim,
My understanding is that the BMW mounting plate that goes with the alloy Adventure top box can be fitted straight onto the standard 1150GS rack with only a small modification required.
I've never done this yet myself but have read about it a few times on here
This thread may have enough info or contacts to sort you out?:-
I have one of those BMW top box plates sitting on my bike right now, (probably) surplus to requirements. (it looks like it might make a neat QR plate for my pelicase that I usually use on there)
I may be persuaded to part with it, but it will be fekin expensive
Do tell Bill - it'd have to be cheap for us northern paupers
(It's a job I'd be sorting on Lee's Adv - Lee of the eastern block ride whom you poisoned with your chilli )