Here's another hobby rocketry 3D model for anyone and everyone to download for free from Thingiverse and print on their 3D printers.

This 3D object is a transition from Este BT-5 tubing to BT-20 tubing. The distance between the two sizes of tubes (transition taper length) is 0.75 inches (19.0mm).

There are three models:
1) Solid,
2) Solid with a 4.0mm hole to allow for screw eyes to be fastened to it,
3) Hollow.

https://www.thingiverse.com/thing:4921365