I like the plan at
www.solarcooking.org/plans/windshield-cooker.htm . Seems cheap, easy, and well designed and a cool idea for camping. We had a "sun oven" for several years, and probably used it 1x a year... In my experience the more expensive ones (a couple hundred dollars) work a lot better than the diy box with tin foil. It's a lot of work to save a couple cents on gas. You have to make sure no dogs, cats, raccoons get into it and the wind doesn't knock it down. And every couple of hours turn it a little bit to get the sun better.
So I think it is cool as an experiment, and an amazing technology in undeveloped countries. For women and children to not have to spend hours a day collecting twigs in deforested deserts in Africa, for children not to die from respiratory infections from sitting in rooms with smoky little fires cooking all day - this is wonderful. But in developed countries, there is not much of an ROI.
If the point is to keep your flat cooler in summer, I think there are better ways. Eat more salads (potato, egg, chicken, veggie - whatever.) Grill in the backyard. Always use a lid on your pots. Maybe a pressure cooker. If you want to make something consider a "haybox" cooker.
http://thermalcooker.wordpress.com/category/thermal-cookers/haybox/ Heidi