I made one of these a couple years ago from some cardboard boxes and aluminum foil. Stuck it out in the sun and the thing got up to about 220 degrees.
http://solarcooking.wikia.com/wiki/%22Minimum%22_Solar_Box_Cooker
Solar ovens are a neat tech. Some variations have parabolic mirrors:
http://en.wikipedia.org/wiki/Solar_oven
http://en.wikipedia.org/wiki/Solar_furnace
The parabolic ones are dangerous though. You put your hand or your face at the focal point of the mirror and you’ll lose ‘em.
There is an even simpler design for the solar cookers than parabolic.
Get a piece of thin, flexible cardboard. Line it with aluminum foil so the reflective part is face up. Then roll it into a simple conical shape.
Cut of the bottom couple of inches, and mount it about the pot holding whatever you want to cook.
Nowheres near as much aiming and aligning going on here. Any sunlight that enters the open wide part has to bounce its way down and come out the bottom hole.