With the later model cars using seat designs built up with varying density foam layers and no springs, couldnt you use the same idea on a custom seat? WalMart sells electric knives that work really good on foam. You need to find a source for the foam material.....use stiff stuff around the perimeter and softer stuff in the butt zone. Stiff stuff keeps seat shape and helps support the thighs. You can use plywood for the bottom plate and cut that out in the butt zone and staple straps from a lawn chair in the opening to support the foam. The foam layers glue together with 3M spray adhesive. Using this concept, you should be able to assemble a seat for a nominal amount....the foam is the highest cost component.