I would call the factory and see if they offer this service? You could get the chambers reamed and have them check the timing and barrel/cylinder gap, crane alignment and all that kind of basic tuning stuff at the same time. In for a penny, in for a pound.
If you are determined to do it yourself, you could call Dave Manson to see if he makes a piloted cutter for doing this by hand? Bob's point about depth uniformity is important, so the tool would presumably have a stop collar on it. It also would likely cost a bit of something, so you would be well-served to have some friends with a similar desire go in on the purchase with you.
Nick