AirHawk. It's relatively cheap and easy - just throw it on when you need it. I beefed up the mounting straps with S-Biners because I read the original hooks can come loose causing the cushion to fly away when a rider stands up on the pegs.
I also scored a Corbin seat recently from a 919 being parted out. It has no cover but otherwise intact. I don't know if I'll ever get around to doing anything with it so I guess I'd entertain an offer.