Weenie. I did mine with a grinder.
Well, I took a novel approach and went with a large grinder with a steel cutting wheel. I used a dremel to make my outline cut and then I went to town with the grinder. Took me less than five minutes once I started cutting. Then went back over the lip with a grinding wheel on the grinder and finally a sandpaper wheel on the dremel. I am pretty sure I have steel embedded in my face, arms, hands, etc. etc. but the keggle kicks ass!!!!
I HIGHLY recommend goggles, gloves and hearing protection. The cut isn't beautiful but I have the edges smoothed out nicely and it is very functional. I am not one of those guys who is going to spend hours polishing the outside of my keg.
I got an empty, serviceable keg for $30!! And to make it even better, the owner of the store knew I was going to cut it up. He opened his back room door and told me to have my pick. I got a Labatt's keg with a blue ring around it. It looks pretty sweet and it is sparkling clean on the inside (after I drained off the quart of old beer).
I have a nice glass lid from the kitchen that fits great. The wife won't notice for a week or so...
I did try to take the section of the top that was cut out and hammer it down on an anvil. Technically this could work but it looked like crap and was a pain in the ass.
I am going to get all the necessary weldless gear from Northern Brew. They seemed to have a better selection and pricing on that specific category when compared to B3.