yeah, craft stores, walmart, etc...
foam sandwiched between 2 layers of cardstock
Use a hot glue gun to put it all together. Get some posterboard to create your template.... it took me a few tries to get it to fit right. Then, I siliconed my corners smooth and sprayed the finished piece with matte black.
Test the spray... it might eat the foam, so I was careful to get as little on the foam at the edges as possible.
Somewhere theres a thread about making a bezel out of posterboard. I can never find that darned thing, but it is the basis upon which I made mine.