A grinder with a diamond wheel, and a wet sponge.

A water Jet Cutter is ideal.

Suggesting million dollar equipment, John? Really? A Fein Multimaster is probably a pretty good tool to own for projects like this. Even the Harbor Freight knockoff will get you started with using one until the name brand can be acquired. It's only around $30 anymore there. Then a nail and a string and some butcher paper for a template for the circumference with the multi tool can knock it out easy.

cool, thanks guys