To fix and plug free end of tube on my filament box, I also wanted something that would grab hold of the filament so that it doesn't go back up into the tube, requiring me to open the box.
Fitting to mount a PTFE tube to filament box:
https://www.printables.com/model/160375-fitting-to-mount-a-ptfe-tube-to-filament-box
The author remixed this model.