Dan L. wrote:

Memories Back in the seventies, my high school days, there was a
project in the Scientific American magazine for making a parabolic dish
for focusing and burning an object. Many art stores sell large sheets of
foil backed cardboard. I cut the cardboard out and made my dish and used
a fine plastic spray from a can that water proofed the cardboard dish.
At night I could put a mike in the center of the dish and hear people
talk at least half a mile away. Set sticks on fire when placed in the
sun. Scientific American Magazine was much better then than now. Today
with internet based news, many of their articles are just junky and
dated before the ink dries. Dish was six feet in diameter and Cost about
$20.

I made a similar dish as a solar cook in fifth grade, around 1968. It's
very hard to get the foil smooth enough to get the focus sharp enough to
use in cooking or to make fires but I did eat a meal cooked on it.