Ross - I agree with Jim. Definitely don't use a jigsaw! Abrasive cut-off wheel has always worked well for me for the rough cut. The idea is to take it a little at a time... grind the plastic. Don't go slow & deep where the plastic is heating up & melting - if you do that the wheel can grab and you have a chance of cracking. Then I like to use a belt sander & then a sanding block. It also helps to make sure the plastic isn't too cold when you're cutting it... room temperature or greater is best to help avoid cracks.