Cutting cardboard tubing
Many moons ago, last century, I used and appreciated a program known as winmiter. It no longer works on today's technological magic, but there's an online free alternative known as Pipe Joint Template with an option to purchase and download an offline version with more features.

The program allows you to print a paper template to mark the cuts necessary to have a clean fit to the joined tubing. The original winmiter program was quite capable and I suspect that this one is as well.
There are other similar options via CAD programs such as Fusion 360, solvespace and I suspect SolidWorks and others, but this is certainly among the simpler methods to accomplish a good fit.