I love it when I stumble across a design that is so simple it's ingenious. Sheet Seat is one of those. Made out of 3/4" plywood the sheet seat fold completely flat when not in use, making storage easy. When you need some extra seating it can simply be opened up. Not only is it a brilliantly efficient design, but producing it would be quite inexpensive. Unfortunately I don't think this wonderful design has ever gone to production.