take a piece of .080 or so aluminum 6 inches square and drill a hole in the center the size of your pilot bit. Then drill 2-4 holes at the outside edge of the plate to secure it to the ceiling. center the hole there you want the speaker and cut away. if you extend the pilot bit as far as possible you will avoid egging out your hole in the aluminum.You could use steel but aluminum is easier to work with. I have one of these [Link: lsdinc.com] but mostly use it in wood,mdf situations. Order a couple of spare blade sets.