said by cowboyro:Cut a rough hole with a rotary tool and a cutting disc. Finish with a file. Works like a charm, probably 30 min total.
This. I've done it plenty of times before, even for round and odd shaped holes too.
Mark out your hole, cut with rotary tool (aka "Dremel") just a tiny bit smaller than the marks, then finish off with a file until you just touch the marks. You'll have a nice looking square hole in no time.