I have a drybox that uses PTFE tube to route the filament to my printer. When not in use, I needed a way to cap the PTFE tube to keep moisture from getting into the 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.
While there are several PTFE tube caps available on Thingiverse, I only found one that also captures the filament. However, it didn't work very well. I created my own design that has some flexible arms inside the cap that will latch onto the filament.
2018-07-30: Updated to fix issue in F360 source file, to reduce wall thickness, and to parameterize a few things.
I printed this all perimeters, but it should also work fine with infill. I recommend using a filament that has some flex to it (PETG is a good choice).