To build a cardboard solar oven, cut a three-sided flap at the top of a cardboard box, cover the bottom of the flap and the inside of the box with aluminum foil, and glue the foil to make it as smooth as possible. Prop the lid with a stick to reflect light into the box.
Cut the top of a cardboard box along three sides, leaving about 1 inch between the edges of the lid and the sides of the flap. Wrap aluminum foil around the flap, and tape it to the outside of the flap. Create an airtight window on top of the box by taping a double layer of plastic wrap on the opening of the box. Use black construction paper to line the bottom of the box. Roll sheets of newspaper, and tape them around the bottom of the box to form a border around the cooking area.
Set the solar oven when the sun is overhead. Adjust the flap so that as much sunlight as possible reflects off the aluminum foil on the flap into the box. Put the food to be cooked on a glass or pie plate, and place it on the cooking area. Place a thermometer inside to check the temperature.