Normally I'll use those little brown cut-off discs. I don't know what their made of, but it looks like basically compressed grit, almost like a thin, fragile ceramic. If you need to cut a wider trench with it, you can stack a few together. These little bastards break if you look at them wrong, but that's okay as they usually come in packs of 30 to 100. They're meant to be pretty disposable. This really is my "go to" tool when working plastic. Don't bother with carbide burrs or diamond bits or ceramic stones or any of that stuff. They're great for other materials, like soft metals or ceramics but they area mess with plastic.
Oh, right - the cutoff discs. Yeah, the thin ones can break easy so get some eye protection. They also make heavy duty cutoff wheels so depending on the cut, you might want to go with one of those.