If you make yourself a jig with some 1/4" plywood and clamp it or tape it down you dont have to worry about the bit walking on you. I didnt bother building a putty dam around the hole instead I just let the hose run on top of the tank. Dont push the bit through the glass ,let the bit do the work. And last but not least... put a towel down so the drop out dosent hit the other side and break the tank.