A very, very simple PTFE tube coupler; just in case you have some PC4-M6 couplings laying around and don't want to buy the official BambuLab ones. Its outside is a 15mm hex shape, so it's easier to screw-in the couplings and attach it to something (if needed)

You need two PC4-M6 pneumatic couplings; these will just screw into this part. The idea is that the two couplers parts kiss in the middle (see CAD screenshot), so your filament won't be “exposed” outside of them. In case you want it extra-safe, you can add a tiny O-Ring between the two couplings.

You can buy the PC4-M6 couplers anywhere, but the linked ones from BambuLab should just work as well