Step 1: Padding
Cut out some closed cell foam to fit the contour of the seat. (I used a scrap from some shop-floor padding, camping pad would be better.) Don't worry about getting it perfect, you will trim it later.
Cut out a slot in the top of the foam. Your *Special* artery will thank you!
Use a dab of superglue to stick the front and rear of the padding to the old seat.