My wife uses the top box as a back rest and has no complaints, but I know there is a pad available that fixes to the top box.
Might not be as comfortable as the gunfighter backrest, but I believe it would not fit without modification like you mentioned.
I will post up some PICs, once I recieve my V46 Topcase & FZ449 kit.
Then, other ppl that have/are bought GIVI's luggage and a Corbin seat w/lady backrest will be able to SEE if it all "Fits". Before they decide to buy!
I Originally planned and wanted to put the GIVI V35s onto my bike, with the Corbin seat w/lady backrest. Spent the extra $228 for corbins backrest. Ouch!
But at the time, I picked 2 friends brains about them fabb'in up a backrest for me and they couldnt come up with any good ideas. Neither could I.
So, for the last 3 months my GF and I, have Enjoyed the V35's and Corbin on several local 1-2 day trips.
It's just now that I realize, that getting a V46 topcase would just add MORE longer trip distance/Fun!
I will report back with the 411, but I might make another THREAD for it.