@Steve I use a solar box cooker in the summer. Temps here are often in the 100's but I've never used it for anything riskier than cookies or bread.

https://solarcooking.fandom.com/wiki/Minimum_Solar_Box_Cooker

@Steve I can't show you proof right now but it is possible to do. You need lots of insulation with the layer closest to the inside being such that can withstand high temps.

The biggest issue is the length of cooking time. Trying to cook a big turkey, for example, when you only get 3 or 4 hours of direct sunshine would be a hard thing to do. However, for something like a medium pizza, cookies or the like it'd be fine.