Seat foam - go to a carpet store and get (free left-over if available) "rebond" carpet padding. Glue with 3M 80 (home improvement store) and trim with electric carving knife (NOT the wife's - you can find them cheap on ebay). Build layers until you get the shape you want.
that turned out well. Are you in Canada or USA? I have used a closed cell kneeling pad you can get from Princess Auto in Canada. It isn't the dense stuff like the puzzle piece mats you can get its firm but also squishy. Hard to describe but like thick neoprene. I made a cafe seat and covered a piece of that with vinyl then attached it to the seat. It was 8 hr comfortable. I road from Moncton, NB to Brifdgewater NS all back roads, 1 stop for lunch and 8hrs total and my but was fine.
This is the one. When you squeeze it you'll see why it's so comfortable. 2 layers of that with a cover would be all day long comfort and still thin and sleek.
Arent a lot of folks just gluing the covers on?You're gonna need to rivet the cover to the pan and then a strip of firm rubber would be a good idea to protect the frame paint from the rivets and also help from vibration transfer.