I do this with plastic to make colored lens. If you find an abrasive substance to work on glass it should work as well. I take a finger screw with a flat plastic head, mount in a drill, or drill press, then hot glue the lens to the large head of the the thumb screw. This gives me rotation to keep things semetrical. The only problem with this method is that you lens is moving, and your abrasive is stationary. This can cause a burn on your abrasive material unless you keep it moving.