We made a huge cardboard box oven for our car camping trips. Only ours wasn't solar. We used a propane stove element for heating and it was great! We made brownies, cake, mochi, even a huge six hour roast in there. With a thermometer stuck in there, we could control and maintain even heating.

One thing we learned was to use a deflector so as whatever you are cooking doesn't get burned.

Total construction time was under half an hour and it lasted about eight months or roughly 30 days of camp use. We used an old broken folding chair as the oven stand. After taking the seating and arm rest panels off, we lashed the legs to fit perfectly around our box oven which was about a 24' cube.

It heated up faster than our kitchen ovens.