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Soldering Iron, with blade attachment. The "blade" is actually an x-acto blade. Most soldering irons with screw-in tips will accept the screw piece from the good x-acto knives, so you can use a razor blade with a soldering iron, and make thin, shallow cuts until you get through it all the way.

Also, you can just use a fine hand file. Drill a hole to start from, then file your way around the thing. Fine metal hacksaw, wire rope, coping saw, etc.

You can also use a "hot wire" as well, but it will take some time, and you'll need air circulation depending on what type of plastic it is.