Made a temporary seat cushion from an old camping sleep mat. This is pretty firm closed cell foam, so it conforms to your shape and is good insulation from cold/hot. It's only about 1/2" thick, but the seat feels really good at that level. When it comes to the final upholstery, I may just have some covers made for this stuff since I can't afford to give up any more room.
Need another piece, but these things are maybe $5 at a camping store.
