I second the freehand deal, following a scribed layout line. I used a 5 gallon pail and a sharpie.
What does a perfectly shaped and sized opening actually gain you for the task that you use the keggle for?
All that is necessary is an opening that you may or may not need to find a stock pot cover to fit.
Hand driving a keg to revolve (maybe 10 rpm's?) so that a high speed steel lathe tool bit held in a cheap bench vise can scratch away at the toughness of a 304 stainless keg is so totally time consuming and cumbersome as to be absurd.
And what of the accompanying dings and dents inherent in most commercial kegs? So much for the hours of setup of 'lathe' and 'machining' since the dents will transfer into the finished cut....
A 1/16" cut off disk on a high speed air die grinder for me, free-handed-maybe 5 mins...and maybe 2 mins with file to suit, as needed.
I've got other things that I have to spend time on!