closed cell neoprene from an foam warehouse (hard to find at a furniture upholstery shop. they use open cell foam). I ended up finding a local manufacturer that sold me a few cut off pieces for $12, enough to make two or three solo seats. It comes in large sheets with different thicknesses and densities. Look for closed cell foam because it does not break down or retain water the same way furniture padding does. layer different thicknesses and densities together with contact spray and shape with sand paper to build the right seat for you.