I'm either going to go with polyethylene foam by the roll (2 lb density, 1/4" thick x 60" wide, $3.59/foot) or in sheets (2 lb density, 1/4" thick, 72" x 48", $21.99/sheet). They are both cross-linked. I think the 1/4" (6 mm) should work. Not sure if the density is correct. I'm afraid 4 lb would...