I just saw this article:
The ingeniously simple design uses two cardboard boxes, one inside the other, and an acrylic cover that lets in the sun's rays and traps them.
Black paint on the inner box, and silver foil on the outer one, help concentrate the heat. The trapped rays make the inside hot enough to cook casseroles, bake bread and boil water…
By allowing users to boil water, the simple device could also potentially save the millions of children who die from drinking unclean water.
We made a solar oven out of a box in Girl Scouts, once, but ours was simpler. We just used one box with foil on the outside and painted the inside black. It sounds like the one in the article would get hotter, though.