If I had the opportunity to improve my solar oven I would do the following.
I would first add more tinfoil, I would add more tinfoil so the box could get more sunlight. Which would heat the food faster
Then, I would make the plate for the food better, so it could always get sunlight.
Last, I would make sure that the installation is neater. This would make sure that the plate could fit in and so that the food inside could be seen.