I make mine out of plywood (1/2 - 3/4)with 3/4 inch cleats and then cover with aluminum flashing. Form the flashing to the cover and staple to the side and cleats. I get 10 years out of them and they are cheap and impervious. You will need some practice to form the flashing corners - it is like gift wrapping - and cut the flashing with a utility knife once you figure out exactly what size you need. If I get a hole in one I staple the hole down and apply a glob of caulking. Simple, cheap, and impervious.