While stumbling thru Thingiverse (an on-line repository of 3D printer plans) I found this OpenSCAD pipe/round duct fittings library. Parametric
at OpenSCAD pipe/round duct fittings library. Parametric by doommeister - Thingiverse
It's a highly adjustable set of plans for printing junctions many of the diameters and angles etc can be changes to generate 3D printer STL files for printing

The blurb lists
This is an OpenSCAD library to generate round ducting / pipe, and assosated fittings. The file will generatePipe
Pipe with an incorporated fitting
90° elbow, Equal T, 4 Way T, N Way manifold, Blanking cap, Single reduction, N Way T
With either solvent weld type ends or a clamp type end.
SCAD suits people that have a programming back ground but if you are not that way inclined ND want a simple custom plan up I'm happy to give it at g.

This is a sample of what it can generate
Screen Shot 2022-04-15 at 6.18.51 pm.jpg

However it can do MUCH more than those shown.

Here's a few I was able to knock up by just playing around.

Screen Shot 2022-04-15 at 6.18.27 pm.png Screen Shot 2022-04-15 at 6.16.21 pm.png Screen Shot 2022-04-15 at 6.15.31 pm.png Screen Shot 2022-04-15 at 6.11.14 pm.png Screen Shot 2022-04-15 at 6.10.38 pm.png

One thing I have not been able to do is have say different size ducting junctions or change teh rails of curvature of the elbow - I'll have to work out how it works and add those in - it will be a bit of a programming challenge

You will need a large printer to print out those multi-arm bigger junctions for 150mm ducting but if you want a custom angle etc it's probably worth experimenting with.
I'm definitely not suggesting this for a complete or even part of a DC ducting system but more if you want a custom fitting or 2.