i used a hole saw for mine. i did 2 , 120mm blowholes in the top of my case about 2 months ago, and it came out good, but i warn you, if you do it while someone is holding the case for you..make sure the hole saw doesn't bite, because it will either cause the drill to try to break your wrist, or cause the case to spin out of the assistants hands. my 11 year old son helped me with mine, and it is soft aluminum, and it took the case out of his hands just once, and caused a slight scratch, but not bad at all. the next one i did was on a steel case, and it was an 80mm hole. my friend was doing the drilling, and when that sucker bit into the steel, he almost got hurt...lol..... his biggest error was the fact that he was pushing too hard, and wasn't completely straight up and down, but he was on a slight angle instead. his blowhole also came out good in the end.
i would like to add, that i was using some very good quality hole saws..but...they were meant for rougher stuff like doing wood. this is why we had the issue of it biting.
if you want to buy a hole saw for the job, i would suggest a decent quality, but either way, make sure it's for metal...the finer teeth are better than large teeth, like mine has.
i would always use a hole saw, instead of a dremel, because the dremel jobs take a lot of time, and are usually a bit jagged at times, since the cut is straight, and you are on a constant curve on the blowhole job.
i would like to learn more on that "punch" you mentioned above...sounds like a hole punch, but bigger than i have seen...but i bet it would work very nice. let us know if you can get more specific info on that, or maybe even a link...that'd be nice....