Right, we ended up not really using our solar box ovens because they can't bake anything nicely if it isn't sweet, and we don't really eat much cake here. Anyway, they are too small for our school feeding over 40 people every day, often 100. An English volunteer recently had fun making cake several times a week, though.
We use solar cookers successfully though, which are called
Scheffler cookers. They are parabolic reflectors made of small normal mirrors for the primary reflector, and aluminum or broken mirror bits for the secondary reflector that is inside the kitchen, under the pot. They get very hot -- when showing them to visitors we like to show off by holding a twisted newspaper in front for a few seconds until it bursts into flame.
These are not ovens, but I can imagine an adjustment putting an oven box instead of the secondary reflector. I think it would have to be rotatable to allow even baking.