I just converted a keg to a boil kettle last week. I found a guy at work (a retired machinist) who had a plasma cutter in his shop at home. For the cost of a six pack of home brew he cut a nearly perfect 12" circle in the top for me. I dressed up the cut with a stone on my dremel tool to the point that it is as smooth as a baby's butt.
I then cut a 7/8" hole with a bi-metal hole saw with the bottom edge of the hole 3/16" above the bottom weld. This gave me just enough clearance for the weldless bulkhead I installed. I installed a Zymico sanke adapter

and a bazooka screen
Over the top of this I also rigged a 12" domed stainless steel grease splatter screen as additional protection against clogging the bazooka screen with hops.
Wayne
Bugeater Brewing Company