Solar oven underway

Thought I'd try and throw together a solar cooker today. Without much planning or gathering of materials, I took some foil and covered the inside of the only box I could find, threw the window in front of it, and put the blackened dutch oven in there. Almost immediately after I put it in the sun at 10am, the temp was up to 140. It climbed to 200 and stayed there. I've got pinto beans cooking now...