Get a wetsuit shop to make a neoprene cover for the tank, strap tank bag over that? Maybe sew magnets of the appropriate polarity in the cover to give the tankbag magnets something to stick to? Just don't get any grit between the neoprene and the tank!
Honestly, though, seems like a bad idea. Maybe fab up a small carrier for a satchel bag on the side of the seat or something, if you don't want full saddlebags?