Hi, I did mine in the lathe as Hood explained, Just grind a bit of Hss tool bit exactly the width you need with a slight angle on the end so the chip curls up as you take a cut. I locked the chuck solid using a bit of flatbar mounted on one of the chuck holding bolts and the chuck guard screw on the headstock. Just wind the lathe carriage back and forth taking small cuts. The whole thing took about 30 mins.
Cheers
John