A nice large bastard type file will do it if you are not in a big hurry. I recommend getting a wooden handle so you are less likely to hurt yourself. Cutting is more a matter of scoring/snapping the plastic but you can also cut it with a saw if you have access. I use a radial arm saw but it tends to melt on the blade if you saw too fast.
Look for ways to support it as it does tend to warp downwards if it spans too far.