Tin foil seems like the opposite of what you'd want -- it'd reflect most heat. Black construction paper would do the best job of absorbing heat.

done it before, I built a dome out of magnifying glases and materials used in a soler cell, then used lenses to direct the heat in the form or a laser to a focal point to cook what ever i wanted. I got tempritures of 560 degrees F wouldent exactly say it was good cooking the first time i tried it, but when you tried frying something using a soler powerd laser that way after submerging it in a water bath, it worked out ok, i might try it again for the purpose of this compotition and cook a curry.