In my earlier sun oven post, I described the basic use of the sun oven and some possible advantages.
It took me about an hour to build the cardboard base. It was a little harder than I thought, if you are going to build one from cardboard, I would suggest that you get the largest single piece you can, as it will aid the overall structure stability of you oven. It took me around 30 mins to duct tape the foil to my cardboard base. This is where you will find out if you built you oven strongly, because the foil adds a bit of weight. I would suggest handling the foil inside, in low light conditions, or with sunglasses on.
I placed my bowl of oatmeal on the oven at 1:55 pm. It jumped up to around 110 degrees Fahrenheit very quickly. I checked it a little before 4:00 pm and my oven was at a whooping 140 degrees Fahrenheit. I stopped my experiment around 5:00 having topped off around 145 degrees Fahrenheit. I tried it with a cover and without one and it did not seem to make much of a difference.
Water boils at 212 degrees Fahrenheit and bringing water to a boil is adequate for disinfection. I believe form my limited tests that if you could bring large amounts of water to boil it could be very useful to have a large sun oven. I would consider a large amount of water a days supply for a person or an entire family. If you could do it for an entire family at once it could be very useful. In a permanent survival situation a child could be placed in charge of watching the water to see when it boiled and could go tell an adult. I do not think it would be much use for cooking because of the time involved. You would also have to live in an area with enough sunlight, which you can compensate for sunlight by using a larger area to collect the sunlight. Another advantage is that you can make many of them and sell them and novelty items or use them to barter with when necessary.