A classic way is to use an old refrigerator or chest freezer. Lighten it up by removing the compressor and the rest of the guts. Solar panels can be found here
http://www.westmarine.com/webapp/wcs...544&storeNum=9
As far as the math, much will depend on the criteria such as temps., the amount and intensity of sunlight, the size and configuration of the box, how often it's entered, etc. I would also want to keep the storage battery(s) warm as that's when they're most efficient.
From my very limited travels to the north in winter, the concept could work in the western mountain area and probably not in the northeast or northwest.