Paper Tube Plug

I use it for storage, but you can use it for shipping. 

Scale-able to fit any cardboard tube.  This model is for a 38mm OD, 33.5mm ID tube.

Protrusions on the side bite into cardboard, or can be a positive retainer for tubes with holes punched in the side.  Could also fit PVC plumbing

Won't rattle loose easily.  A twist using the middle web eases removal.

Created using FreeCad 0.21

Example printed in draft mode (.2 @200mm/sec)