Those rotary drywall cutters are the ticket for cutting plastic. I first used one when I was a mechanic before changing careers, and we used one for trimming tough plastics like bumper covers. Takes a bit of practice to get a nice cut, since they are super aggressive, but they make a very nice, clean cut that's far better than any other tool I've tried. I've actually never used mine on drywall, I prefer one of the oscillating tools or a jab saw, but I've used it quite a bit for automotive and electronics work. You can make a front panel for a project stupidly quick with one of those, and it turns out nicer than cutting with a dremel cutoff blade.