The tolerance for the countersinks on my current project is pretty tight. I have 20 holes to drill and I want the countersinks to be consistent. None of the depth stop collars that I have will work, so I made my own:
I can center the washer and fitting over the spot I need to drill, insert the drill and when the fitting begins to spin, I know I'm at the right depth. The washer prevents the copper fitting from marring the wood when it spins. You'll notice that I had to shorten the fitting just a little to get it to the right length.
Feel free to steal my idea, I don't plan to patent it ;-)