http://www.hostelshoppe.com/cgi-bin/...ory=1089843061
This is called a hardshell seat, often made of carbon fiber or fiberglass, with a pad or cover. The link above is one example. These things cost anywhere from $100 to $310. I don't see that much resemblance to a snowboard, but if you do, well good for you. Someone else in another thread thought they looked like a banana. It might be about as easy to make one from a banana as from a snowboard, but I think I would just carve the shape out of styrofoam and then cover it with fiberglass if I couldn't find one to buy.