I drilled mine with a dremal and dimond tip tipped bit, the ball shaped one worked best. I just draw the circle on the inside with a sharpie, then with the tank upside down on the bottom I used clay/plumers putty/ sticktack/duck tape to build a little dam around the circle I want to cut. I filled the damn with water to cool the bit constantly. Then just make tons of passes around your circle until you cut through it. Good times.