I have had reasonable luck using the Greenlee "High Speed Cutters." In technical terms, these are "rotary broaches," rather than hole saws.
Nevertheless, in replacing some panels I have had to resort to removing the bottom of the old panel and attaching it to the new panel. Or, when I simply need to place a small hole where there is a big knock out, I will make the "small hole" in a blank 4-sq cover, and attach that over the large knockout.
In either situation, pop rivets are my fasteners of choice.
By now, I sometimes feel that, if you're not fully equipped to make holes for pipe in a variety of materials- maybe you shouldn't be working on panels! A pox on panle knock-outs!
