They can be insulated with several layers of dead air space, crushed newspaper, sawdust, etc. Use of Cooker use a dark cookie sheet or other baking pan to hold food and catch boil over. Cook in dark pots or painted glass jars and lids. Poke a small hole in the lid to relieve steam pressure. These ovens cook in winter and summer.
Bright sun works best.
Hazy days are okay. Start cooking early in the day. Set the oven ahead of the sun approximately 1 hour. More food takes more time, less food, less time. Use typical water ratio.