How cheap you talking? I just made my own out of mirror backed acrylic, but I see you don't have any.
If it were me and I really didn't care what it looked like(as long as it worked), I'd look around the dump for some random plastic. Could be the bottom of storage bin, a big tupperware container, whatever. Heck, even a piece of thin plywood, or luan would work for a while.
How big of a piece do you need?