I agree with JW in starting with a ply base. A few other points that we use to make a commercial grade seat.
1. We only use hypalon for the cover. Costs a bit more but will last as long as the tubes !!
2. We use a closed cell foam that we purchase in any size from a company called Sherlock Foams (on net) and supply by carrier.
3. The ply base is good quality 3/4 ply. The foam is bonded to the ply with sikaflex. The foam edges can be trimed for softer corners.
5. The hypalon is fixed taught (heat gently first to allow for warm weather expansion) over the foam and is sikaflexed underneath with galvanised staples holding it inplace until the sika goes off.
6. As JW notes the ply can then be screwed from underneath.
Not the softest seat in the world but does the job for a 1001 bums, takes a pounding and NEVER absorbs water.