The traditonal method of mounting a sidecar calls for a machine with a full frame. You dont have one of those.
The traditional method of mounting a sidecar to a 1150GS is to stand back and keep throwing $$$$$$$$'s until the result is achieved.
OR, get yourself one of the new Urals, set up with a chair attached, and keep the GS unsullied.
Is this extreme, not if you do a full and realistic costing.
Years ago, a mate put a chair on a K1100LT. The bike cost $10K at the Police auctions, he stopped counting when he hit $20K. Eventually sold it for $12K. It was well sorted by then too !
Outfits eat rear tyres, the best fix is to fit a 15 x 5 wheel and use 15 x 165 car tyres.
To have a properly set up outfit, you will render your bike unuseable for anything but outfit work. You cant have your cake........
and that, is my $0.02 worth.
gregGS