I understand your concerns about moisture, but the spar urethane on the masonite offers excellent protection. It only costs a couple dollars for a small container at the hardware store. The treated masonite worked great for me. It cut easily with a jig saw. I like the rigidity when stretching the cover, and with the spray adhesive and staples it held the cover well. I personally like it more for this purpose than the foamed plastics. As for weight, it depends on whether a pound matters to you.
