I use to do mini bike and go kart seats. The mini bike seats were usually 3 to 3 1/2 inches high. I found rebound foam made a great base. It is extremely firm. You would build up about 2” of rebond and the next layers with a firmness to your likes. Btw, rebond is the foam used under rugs. This maybe what you call chipped foam. It works use it again.