To answer your question, yes. There are square BROACHES. Any industrial supply house (machine shop supplier) will have access to them.
To offer my opinion, as an ex-tool and die maker, drill them round and then file out the corners with a tiny file. I did 12 holes (a cocktail with three cp's) in 1/16 metal cp's in about 15 minutes. It's very simple.
Good luck,
John