The reticulated foam or "drain dry" gets my nod. With a perforated or mesh cover water
runs right through. That way your not sitting in a puddle like closed cell and vinyl covers do. Its pricey. Look to Rochfordsupply.com for online retail canvas
needs. their prices beat most other retailers by a long shot. They carry a good foam selection too.
Regular foam doesnt do so bad if price
is the main concern. Use a solid vinyl cover material and a mesh bottom.