This is an improved version of my previous design I have made ( https://www.instructables.com/id/Box-Solar-Oven/ ). For this you will need two boxes; one smaller than the other. You will also need an adhesive (I prefer hot glue), scissors, foil, plastic wrap, and some type of what ever you want to use insulation (paper is an option).
Step 1: Measure Your Boxes
Take your boxes, and put the smaller one inside of the larger one. There should be a bit of space between them.
Step 2: Cut Off Flaps
If there are flaps on you boxes cut them off. Also cut the top of the small box off. Do NOT cut the top of thee big box off.
Step 3: Put in Paper
Take some of what ever you want to use insulation and put it in between the boxes. This will act as insulation, and will help retain heat in you box.
Step 4: Cut Top of Large Box
Cut the sides of the larger box into slants.
Step 5: Cut a Hole in the Top of the Box
Close the top of the box and cut a hole in it to line up with the smaller box.
Step 6: Cover Top
Take your plastic wrap and cover the top hole. This will help trap the sunlight.
Step 7: Cover Inside With Foil
Take the foil and cover all of the inside of the inner box.
Step 8: Help Reflect Light
Take some extra cardboard and cut it into 4 small rectangles and cover them in foil. Put them in the corners to direct the light.
Step 9: Testing
For my testing I put it out on the sunniest day I could get (partially cloudy) and put it in the sun. It got up to 140 degrees (Fahrenheit).