I got a free old dish, approximately 80" around and covered it with stainless steel license plates. The license plates came from a scrap metal dealer.
Once the dish was covered I leaned it on its mount and hung a pot of water in front of it. It had steam pouring out in a couple of minutes.
I put the dish on the mount and can turn it to track the sun. I think any dish over 36" would be usefull. Aluminum foil could be used but would not last as long as the stainless plate. I expect this to last at least 20 years. These plates were left in standing water for several months and never rusted.
Cast iron cookware should be used because it is very hot on the side closest to the cooker and the cast iron will heat more evenly.
Good sunglasses or welding goggles are a must and you must be very carefull not to walk in front of the focal point. It can set you on fire before you can react. Children must not be able to access it, this is one reason I mounted it higher than I would like to. The focal point is over 6' off the ground and this keeps it off of any stray kids or onlookers.