I made one of these a few years ago. I stretched aluminumized Mylar over a round 2 foot pizza pan. I put an air fitting in the bottom of the pan and attached a squeeze bulb to it so I could pump a partial vacuum beneath the Mylar.
The resulting focus-able concave mirror worked well for catching paper on fire at 50 feet with sun light.
There was a book by an electrical engineer called, “How to Turn A Microwave Oven Into a Ray Gun”.
He used a busted 5KW commercial oven’s innards to set plywood on fire at 500 feet. Boiled a cup of water in 4 seconds.
Just think of what would happen to the operator if the beam reflected back!
I’ll bet this stuff is on YouTube now.