Inspired by the Estes Tube Cutting Guides, I needed something for BT-70 and BT-80 tubes. This OpenSCAD file generates suitable STLs for larger tubes. Although you can use this to make smaller guides, I recommended getting a set of the Estes guides as they are a great product for the smaller tubes.
It is possible to get a smoother finish by increasing the $fn value
Simply move the top guide to the location that you want to cut.
Slide the second guide until snug to keep the assembly from moving while cutting the tube.