I picked up a keg recently to turn into a keggle and the person I got it from loaned me their cutting jig.
Unfortunately, the keg was a miller keg and it had the threaded neck and the jig wouldn't fit. Not to worry, I've always got enough crap laying around the garage to improvise (it really pays to not throw stuff away).
Behold my improvised keg cutting jig:
A doorknob, a few pieces of 2x4, some scrap wood, and some duct tape, easy peasy!
Dents and dings aside, it did a really great job:
Add a nice BobbyM sightglass/thermo and a ball valve that I had bought a year ago and viola! Keggle!