3D print an end cap for round tubing
For round tubing, diameter is the key measurement. An internal plug uses the inside diameter; an external cap uses the outside diameter. Real tubes can be slightly oval, so measure in more than one direction.
Check roundness
Measure the diameter in two perpendicular directions.
Insertion depth
A very long plug can create unnecessary friction and become more sensitive to tube deformation.
Flexible caps
TPU can be useful for softer protective caps.