This is how you can make an affordable and a working solar oven that'll get the job done.
Step 1: Set It Up
Cover the inside of the shoe box with aluminum foil to reflect the sun light/heat
Step 2: Seran Wrap
Cover the opening of the solar oven that way it keeps the heat insulated inside of the shoe box. This also helps to keep insects out of the box.
Step 3: Set Outside
Set the solar oven outside where the sunlight can enter from the gap you left open and allow the food to heat up/cook and enjoy.