I was looking to do same. Ended up removing seat but keeping grab rails. I now just put my bag on the fender between the rails and strap it down. Can put a piece of foam down first if worried about scratching (the bag I use has a padded bottom so not a problem). The ideal though would be a flat plate screwed into the four, capped holes on the grab rails themselves. When I realised I would have to get something fabricated I went the route described above. Not unhappy with it tbh.I would like to replace the pillion seat with a small luggage rack if you have an idea about that.