This is a simple adapter to allow fitting two tubes together, where both tubes may have different sizes. The design is in OpenSCAD and it can be modified with the OpenSCAD customizer.

This is an independent design, but it is quite similar to the Parametric Pipe/Rod/Tube adapter. The main difference is that the sections that fit on the tubes, can be slightly tapered. This makes a good fit more likely, and also allows for some tolerance.