I only needed the depth of my tallest cooking vessel plus a baking stone and drip pan to decide the depth. I make my cookers 9″ deep. The goal is too have the shallowest cooker possible to get the hottest temperatures. The shallower cooker has less area to heat up so it heats up faster. It helps to bring cold food up to cooking temperature faster.
If you have noticed my oven appears shallower when cooking pizza, that is because I prop up the floor with empty soup cans or another cooking pot so the pizza is closer to the top of the oven.
Heat rises so the hottest place in the oven is the highest. Direct sunlight is the cooking heat. Your solar cooker only works in direct sunshine. You MUST not allow the food to get shadowed.
Once the food reaches cooking temperature it can still cook during cloud coverage as long as the solar cooker gets sunlight at about 30 minutes each hour. It will continue to cook on stored heat from the heat sink. The baking stone is a fine heat sink. A better insulated oven will do an even better job of continuing to cook during cloud coverage.
I once started to cook a pizza crust, then it became it became cloudy and rained! Then the sunshine returned. This is typical weather beginning in October through April. A pizza crust normally takes two hours to cook on a clear sunny day. That day it took four hours! It was not a problem because it tasted great!
The only wasted food I have had was when I over cooked potatoes. They are the easiest to solar cook quickly. I forgot them and they were left cooking for hours after they were done. I do not attempt to cook animal products on a marginal cooking day. A marginal cooking day is a day with any amount of clouds.
On an overcast day when my oven doesn’t reach above 150°F, I have successfully baked potatoes. I place them in a lidded canning jar. The canning jar works a lot like a pressure cooker and speeds up cooking on a regular day, or makes it possible to cook on a mostly cloudy day.