Just the thought of experimenting with aluminum foil sent me searching for solar cooker designs. A solar cooker is essentially a reflective dish used to focus sunlight to cook food. Commercially they come in many different shapes and sizes but with a bit of geometry I think we can optimize a DIY shape for the best light collection. My first instinct was to use a parabolic reflector. A circular paraboloid can collect a large field of light and redirect it in to a focal point. This allows for far higher temperatures at the focal point than at any other point in the field.
The only problem with this shape of cooker is that it must be positioned to directly face the sun for optimal temperature. Theoretically the solar panel amplification dish design below could be used to create a solar cooker that would collect light even as the sun moved over time.
(via fossil freedom)