Wow!! The first one I built, that worked pretty well, was cardboard covered with aluminum foil. Second one was aluminum flashing that was folded up into a reflector oven. $278 and $99!!!
In the '70's, when there was an energy shortage, my school kept reducing the thermostat to save fuel. My students and I took 4' x 8' sheets of cardboard, covered them with black paper, and hung them in our classroom's south facing wall of windows. The maintenance men would come in, take a reading, and lower our thermostat, but the temperature in January was still +76º. They returned and adjusted until they had the heater shut down and we still stayed warm. Those 6th graders were so enthusiastic for solar heating, they built various solar ovens and we cooked lunch!
Bob & Donna
'98 Gulf Stream Sun Voyager DP being pushed by a '00 Beetle TDI