This is something i have not checked out yet, but it may help someone out in this age of recycling everything
I was told that a good supply of Aluminium sheet can be got from an old freezer or fridge outer casing if you don't mind a bit of recycling, but removing the Freon gas is the problem, bad stuff to mess with.
and the inside is a good source of plastic sheet, but I'm new to boats so don't know much about the materials normally used in their construction.