The easiest, most sensible, and most affordable solar powered oven I have ever heard of was recently a winner of the FT Climate Change Challenge. Jon Bohmer sat down to work on a project with his daughters. All it required was two cardboard boxes (one inside of the other), an acrylic cover, black paint, and silver foil. With the boxes inside each other, the box on the inside painted black and the box on the outside covered in silver foil, they create an oven beneath the acrylic cover. The box is said to get hot enough to cook casseroles, bake bread, and boil water.
Solar Box Cooker Instructions: Educational Green Project