In a previous thread ( http://forums.bmwmoa.org/showthread....&threadid=2538 ), lorazepam showed how he mounted a topcase on his RS. I was so impressed by his pictures I bought a topcase and mounted it easily to the Craven rack on my R75/5. However, I traded in the old bike for a new RT, which has a rack with a shape different than the Craven and the RS rack.
As Loraz noted, one of the more difficult parts of his project was finding the plastic that would level the case. Unlike the RS rack, the RT rack has high raised lip at the back. I figured a large flat piece of black plastic would work, but where to find it?
After looking through home centers without success, I found what I needed at K-Mart - a black plastic cutting board (Martha Stewart EveryDay, no less).
I placed the board with the drain grooves down and the handle facing forward on the rack, making sure there was enough front clearance to remove the saddle. Then I drilled up through the rear rack slots and test-fitted the board with two #10 x 1.25" bolts. Next, I removed the board and the rack (6 Allen bolts), reattached the board, and drilled up throught the two front slots. Off with the board, on with the rack, and reattached to board to the rack with a total of four bolts. It was a simple task to drill the four holes in the board to attach the topcase mounting plate. The handle in front is a convenient place to attach bungee cords when securing something to the rear seat.
The board is both longer and wider than necessary, so you may want to trim it to fit.
Thanks again to Loraz for the original idea, and now to Martha!