There are inexpensive plastic cutters available, and it would be trivial to make one if you have a grinder or Dremel tool. Here's a picture, though not close up:
http://www.pictureframes.com/html/plasticcutter.html and another
http://www.dickblick.com/zz574/77/
I've seen them for a couple of bucks on occasion. It's basically a knife blade with a bevel at the tip that is dragged so that it machines out a small curl of plastic as you go. Cut about 1/16" deep and then you just bend the pieces and they break at the line.