Ive done it using water a couple of times, it can be a bit of an art jigging it up at the correct angle. Not very practical for big lumps.
Lasers work well, providing you can lay it all out well enough to project the line. Used that method on some really awkward shaped sections before.
The template/measurement way works best though, and can be done in virtually any positions. Like Paul said, you don't need a template, just mark it on the tube. 3m tapes are good for wrapping around stuff tightly, as they fit around tightly and dont try and go back straight like bigger ones.
Easy way: score the pipe at the angle you want with a grinder at both ends of the cut then lay a piece of string then spray paint over it- it will give you a nice crisp line to follow with thin cutting discs ! Same method was highlined by another member in a post above...it’s the way i’ve Done it many times ! Works !
I went for the string method, which, while easy, frankly isn't very good. The string doesn't want to follow the correct route, it wants to follow the easiest, which isn't the same as a true section cut.
However, it got me close enough to then put the cut section on my bench and, with an appropriate shim under the pen, run a marker pen around the edge of the drum to get a nice, clean slice through.