I need to cut a few bits of 6” plastic duct, is there an easy way to do this?
(Trying to get the ends squared using a freehand hacksaw does not look easy.)
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.
I used a rubber band that I put round the duct; once I had the band in place I draw a line round the duct using the band as a guild. Then I used a hacksaw.
The fittings give an overlap of about 3cm, so the cut does not have to be spot on.
I also got some duct that would fit tightly inside the main duct, using a bit of this and two bits of the normal duct, I make sections that could be adjusted to the correct size in place.