Here ya go, Mr. The Mole!!!!

This is a model of a BT-50 to BT-55 Tube Adapter and is designed to fit Estes BT-50 and BT-55 airframe tubing.

There are five designs to download with varying transition lengths.

1) Transition length is 0.5" or 12.7mm
2) Transition length is 7/8" (0.875") or 22.23mm
3) Transition length is 1.0" or 25.4mm (similar to Estes TA-5055, Semroc SEM-TA-5055 and Balsa Machining Service TA5055 tube adapters)
4) Transition length is 1.5" or 38.1mm (similar to Semroc SEM-TA-5055L and Balsa Machining Service TA5055L tube adapters)
5) Transition length is 2.0" or 50.8mm

The 'A" versions are solid while the 'B' have a 2.0mm hole in the center that runs the length of the object.
https://www.thingiverse.com/thing:4744003