My helper thinks I am a genius (which is just as good, because the feeling is mutual). I actually cooked an egg using an old cardboard box. It worked so well that I shall be using my cardboard box to braise meat on sunny days. I will post more when I have done it.
It is the box left over from the purchase of a Brother printer.
It needs a black cooking utensil to absorb heat.
It needs a black cover and a glass heat trap, to trap heat.