This video shows a way you can make a mold for big cardboard or plastic or sheet metal solar cookers. I have made one solar cooker with this method. It is a compound parabolic reflective dish and it keeps the reflected sunlight in one area for up to 3 hours.
If you want to try a a similar dish, without making a mold, I may put a template (made from my mold) online soon. Comment if you want it.