I use a dremel with a cutting wheel. Drill the corners, then use the dremel to cut the straight lines. Run it slow enough so it grinds out the plastic, not melt it. Finish it off with a straight file for the straight edges, and a round file for the corners. Or just make the corners square if you prefer. It takes practice, but the results are not too bad.
In the past I've used a soldering gun with plastic melting tip. It works, but it's harder to make straight lines.