There are a lot of different instructions and ways to make solar ovens, but it really doesn't have to be complicated.
Put the smaller box inside the larger box and loosely stuff newspaper in between the sides and under it if there is room. Seal the top edges in between the boxes to hold in the heat by folding the flaps of the smaller box out over the bigger one and then cover the four corner holes that there will be with cardboard. Attach a lid/flap on one side (or more than one side if you like) with aluminum foil on it to reflect more sun down in the box.
I had some ceramic tiles lying around, so I decided to paint them black and put them in the bottom as they retain heat. I think they helped a lot, but you don't have to use those. They also made channels under the pan so hot air could get under there. You can use a rack for this.
(Oh, it's a good idea to get a meat thermometer to use as there is more guess work in the cooking times unless you just want to cut into it whatever you are cooking to check it.)
I'll be cooking different things this summer and will post temps, cooking times, and photos.
Update: I cooked a Cornish hen...and it was yummy! Some clouds were coming through, so I didn't get the best cook time possible. It took 5-6 hours (75-80 degrees on this day), but I will do another one soon to get a more accurate cook time on a sunny day. Still pretty cool when you can put a bird in a box outside and have a yummy tender dinner, using no energy and not heating up the house on a hot summer day.