RKA way to go!
The folks at RKA will solve your riddle and in the color of your choice. My '87 K75S was well served by one of their smaller bag with the three point hitch. Be sure to have the velcroed map case as there are times you'll really want to change its orientation.
Don't get a Chase Harper. Floated all over the place on my K75S and velcro would open at high speeds (small fairing). Lost all my paperwork for a cross country IBA ride. Really pissed me off.
I have a BMW bag with a 6" zipper at the seat. The bag doesn't float at high speeds cause its too stiff ( I have a somewhat larger fairing & that helps)