This thread is cracking me up, but on a serious note... If you don't want to file your round holes square (which as we've heard several times is the wrong way), or you don't want to start with round holes and make them square with a file (also clearly wrong) you could use a square hole Punch as suggested by Hypernova, only problem there is his source doesn't provide stout enough punches.
We use Greenlee punches at work for our square holes in steel needs, and I think I have seen people discuss similar products for getting a button sized hole (round) into a metal CP also. You can use them with just the punch and a wrench.
WARNING, this is a rather large PDF of the catalog, if your not interested in the punch I wouldn't wast the time or bandwidth.
http://www.greenlee.com/cat_docs/Holemaking_07.pdfEnough serious suggestions, has anybody tried cutting a round hole and filing it?
Later,
Brian