These are very useful for drilling round holes in plastic, http://www.amazon.com/Neiko-Titanium-Step-Drill-Bit/dp/B000FZ2UOY/ref=sr_1_1?ie=UTF8&qid=1342737118&sr=8-1&keywords=unibit
When the hole is not round (ie. square) I drill a round hole and then use a file to square it up. Doesn't take long and given a little practice produces nice results.
Both tools work with plastic as well as sheet metal.