Mate i,ve done a similar mod to my aero,s seat except i used a linishing belt on my seat and i took out a good two inches of foam, as for the cover looking like the top of a drum you can have a curved seam sown into the cover or just add  a couple of upholstery buttons to the seat and poke the string thru the base tie off and glue, just as quik note also glue a reinforcement patch on the underside of where you put your buttons at the fraction smaller size and don,t place them in the main pressure points of your backside.