whatever your particular big one is:
http://www.treehugger.com/files/2009/04/...e-challenge.php

One cardboard box fits in the other one, aluminum foil, black paint, and Bob's your uncle! The one shown cost seven bucks, but I'm not sure how the spent that much.

I have a friend who dotes on solar ovens, but he spends hundreds of dollars on commercial ones. I know from eating his pizza that they work.