I built a couple of these types of items back when I was a teenager; one was a reflector box using smoothed out tin foil and carboard and the other was a large parabolic reflector. The reflector box got REALLY HOT when it was pointed in the right direction, it just depends on how much reflector surface you incorporate. Not sure if I actually did cook anything with it though.

The parabolic -- well let's just say "not so much". The curve wasn't exact enough, I could put my hand at the "focal" point, it was warm but not bad enough to make me need to move the hand. It was supposed to cook hotdogs according to the book!