I have Scadden Rampage, and I want to build a rigid deck for the area behind the seat. The deck will mostly be for my dog to ride on, but I also want my daughter to be able to ride in a folding chair, and I'd like to be able to stand up (or at least kneel on it) to fish from when anchored in a lake.

Light weight, rigid, waterproof, non-slip...anyone have ideas besides good old plywood and varnish?