TPU end cap. Designed to fit PTFE tubes with 4mm outer diameter. The inside is tapered so the further it's pushed in the more friction is applied to hold it in place. Works well for me let me know if it works for you.

If you like this design, maybe you'll also like my Ultimate Filament Holder?

Edit: I created a newer one located here: [https://makerworld.com/en/models/1629574-ptfe-plugpal-tpu#profileId-1720900](https://makerworld.com/en/models/1629574-ptfe-plugpal-tpu#profileId-1720900)

Functionally it's the same, it just adds a few ribs to help improve grip.