[DonRotolo]

If you want the mirror to focus, it needs to be parabolic, not spherical.

A sphere or a section of a sphere can be made with a suitable ball, or balloon, depending on the size you need. Up to 2 or 3 feet, use a ball, for up to about 8 feet (diameter) use a weather balloon. Coat the form with something that will hold its shape for the size - paper mache, polyester or epoxy resin with fiberglass reinforcement, or similar.

For a parabola, a very good estimation can be made by hand. Get a piece of paper large enough to cover the radius of the mirror, and graph a parabola (X = n*y^2, where n is a scaling factor less than 1 that is used to flatten out the parabola enough so it looks like a dish and not a deep bowl.

Once you have the proper or desired curve, cut eight (or more) pieces of stiff cardboard (corrugated, or maybe foam-core) and arrange them around the center, so they form a framework of a parabolic dish. Then fill in the spaces between the 'ribs' with thin cardboard. Use glue and/or tape liberally. Then cover the thin cardboard with your reflective material. More ribs makes a better reflector.

If you need an optical quality mirror - almost good enough to be a telescope - then make a form as described above, and then spin it at a very constant speed (There's a calculation for it, but I don't have it handy. It was in a NASA Tech Brief from 1983-86 sometime). While it is spinning, pour in epoxy resin. The spinning will cause the resin to level out, but instead of flat, it'll form a parabola, within a few thousandths of an inch. Keep it spinning until it hardens, then coat it with your material. Or, create a negative form and use that to make as many copies as you like.

Let us know if this isn't enough. Also tell us how large, strong and accurate it needs to be. What are you using it for?

Don