High Density Polyethylene Sheet 3/4" thick, 4'x8' in white. This was left over sheet, from a boat project.
This was sourced from Plastic Supply of Maine in Biddeford.
Sorry about the small black/dark gray dot. That was caused by the exhaust on my snowblower, I moved it after it made that mark. Oops. There are no holes in it, those pieces of wood are just holding it against the wall, because its heavy.