N's solar cooker

This is a picture of N's solar oven. He made and designed it himself. In our hottest weather (which has been pretty hot), we can only get it to 145 °F. I made one with two layers of boxes and newspaper insulation between them. I followed someone else's instructions, but I used much smaller boxes since I did not have any as large as it called for. The best I could do was get mine to 162 °F. Supposedly they can boil water, so I'm thinking the extra size must matter, or else I did something else wrong when following directions.