How do I cut plastic duct?
If you're going to use a hand saw, you need a mitre box really. It's way too hard to cut square by hand, because if you start at even a bit of an angle, the saw is going to want to follow that cut.


If you have a mitre saw that is big enough, that's definitely the easiest.
A table saw may work in a pinch, if you use the sliding guide rail.
Make a couple marks using a measuring tape, then draw a line between them. Using a square is probably easy, or using a chalk line or piece of string can work too. You should then be able to use a jigsaw to follow the line and make a pretty even cut.