I received the parts to be machined and everything went better than I thought possible. I only ruined one part and that was because I pressed the button while the drill was still in the hole, preventing the stepper from turning. I was lifting the spindle, so the stepper turned only a portion of the 60 degrees. I discovered the mistake when the last hole ended up too close to the first one.
I made a quick drawing of the tooling in the vice:
And the firmware for the Arduino is in this Github Gist. It's really only the sample code from Rugged Circuits changed slightly for serial debugging and with values appropriate for this stepper and the 24V supply.