I had a wrench from some harbor freight tool that was little more than a thick piece of sheet metal. I'm thinking it was from an HVLP gun or the pneumatic rivet puller, but I don't remember for certain. Anyways, it fit behind the chuck perfectly and by putting the allen wrench in the jaws it came off without a problem.
Having said all of that, I wish you better luck than I had. I switched to a rohm chuck and the bits would loosen up in short order and fall out so I ended up going back.
