Hi
I need to make 50 cm long carbon fiber octagonal pipe 50mm flat-flat with wall thickness of 2mm, outside surface quality is important. Im looking for ideas how to make it. Currently the best solution i came with is to do vertical vacuum infusion with heat formed pvc pipe as outside mold. If you have any ideas, guidance please share.

Depends on the geometry. If it is a simple straight octagonal tube, then I'd just get some octagonal aluminium extrusion, split it with temporary barriers on opposing edges, and make the mould as per Warren's linked tutorial.

If the pattern was less simple, and for some reason octagonal extrusion wasn't the right shape or the tube needs a bend in it, then I would build the pattern by hand in the traditional way with foam and guide templates. I do the same thing making custom intercooler piping for very complex pipes, and there's generally about a days work in getting the form right. An octagonal tube would be even easier than a round tube for this, as you don't need to try to sand an even round surface using flat tools.

I'm yet to discover something that can't be made this way. Don't get me wrong, CNC's are wonderful and can make your life a hell of a lot easier, but I'm also lucky enough to own one. If you don't have access to a CNC, and you aren't looking for volume production or engineering tolerance accuracy, then making stuff by hand is perfectly suitable.