I have run across a very interesting topic, about a box-type sun cooker, and decided to do some research.
I found some interesting sites about types of cookers, construction, and cooking methods and recipes. The box cooker in the other post I read, sold for about $230. Looks like you could build your own for less than that, or perhaps the same cost, but better construction.
The Solar Cooking Archive :
The Solar Cooking Archive
The Solar Oven
What I liked about this design was the access door in the back. This one uses sheet metal, but likely could be adapted to high strength cardboard, or masonite panels.
The Devos Cooker - nice tabletop cooker
I could not locate detailed plans for this, but looks easy enough to extrapolate information, and make your own plans
Concentrated Parabolic Cooker
This one, beginning at mid-page and on to end, is based on the Devos Cooker principles, but offers a more simplified method of constructing the reflector
Superior Reflective Materials :
Reference > Reflection > Miro-Sun material
The critical point, with the parabolic cookers, is to test for and locate the focal point of the radiation, and position the pot there.
The box-type cookers seem better for baking, while the parabolic cookers seem better for grilling and frying.
With any of these, care needs to be taken with not catching "flashes" to your eyes when in use. This seems to be alleviated by approaching from the backside of the reflector, positioning yourself in the Sun's path, or using the table top parabolic design. In any event, dark sunglases when cooking are recommended.
I would imagine oven gloves as well, and taking care with not getting hands in that optimum heat focal point.
** On Pages 12 - 17 of the document at this link, there are detailed plans to build an inexpensive, very portable solar panel cooker.
I think this setup could be a really great one to get started with :