Wecome to the board!
I don't know if you have a dremmel but I used the 1.5" EZ lock cuttoff wheels for metal. It took 3-4 wheels and a set
of ear plugs from work. A 5 gallon bucket placed on top of the keg, use a sharpie to outline the cut, and instead of
going all the way through, using the sharpie line as a guide I went about half way through the top, creating a groove.
That way, it seemed, the wheels lasted longer and the final cut takess less pressure.