I made me a jig for cutting tubes on my table saw. It works to cut tubes square and also was used to trim the switch band on my av-bay after I glassed the band that was already installed. it's versital, but you have to build it right.
The 90 deg pieces that the tube sits in is easy. mounting it to the flat board, takes a little more, BUT the real key is to make sure the oak (or other hardwood) piece attached to the bottom of the jig that fits in the guide in the table saw is snug and mounted so the tube will be square to the saw. The higher you put your blade, the more critical this becomes.
I actually trimmed the end of the jig after the guide was attached.
I cut a piece of 1/4" scrap plywood to the width of the tube. Since the faces are 1/4" wide and square, it has to widen the tube in that direction to fit in. This helps the piece stay in place and I use tape on the inside of the tube to keep it in place. A round head screw in the center of the end makes a single pivot point against the fence so any non-square ends of the tube won't affect how it is rotated. I make sure the piece is longer then where the tube is being cut. That way it stays in the part of the tube in the jig, but also keeps the cut off piece from rolling or flying off.
Here I was just trimming the end of the tube to square it up.
This jig works best when just trimming ends, but if you do a rough cut with a hacksaw, it will square up the end very nice.
I've used this on tubes from 2" to 6" to square ends which I believe is critical for high G HPR when you have booster tube pushing on a switch band and the band against a payload tube.