top is plywood
Ed is correct, the top is nothing more than plywood, 1/2" as I recall. With the two lids, it's not very efficient.The sides and bottom are about 4" all around.When I tore mine out the bottom was somewhat waterlogged, which had to hurt it. I didn't spend any time trying to figure out how that happened as that entire part of the boat went into the dumpster.Insulating the top should help a lot. If you are not going to rebuild it, simply gluing a 3/4" or thicker sheet of foam up under the lid should help a lot. If I were doing this, I would seal the back opening, then try to cover the underside as tightly as I could.The other big drawback is the odd shape of it, making it difficult to use it effeciently, but that will take a major job to resolve.