Summary

I don't like the bowden capture units that you buy, they are ok if they work and you don't want to remove them but are a pain if you want to swap the tube as they cut into the Teflon.
I tried some of the offerings on Thingiverse (those with screw on tapered nut designs) which I found too flimsy, especially for the M6 version. This design allows easy removal of the tube without damage.
If you need to join two bits of tube you can make a cone on the two ends and use two of the top clamps.

Method:
You need an M6x30mm insert from Aliexpress et al. or you can get an M6 bolt, heat it up to soften it and drill a 2 or 2.5 mm hole in the centre (probably with a drill press).

Take your PTFE/PFA tube and make a cone shape on one end by softening and shaping the tube on a hot nail.

Screw the M6 bolt/insert into the bottom part and into the extruder (clamp if necessary with M6 nut(s).)
Slide the Teflon tube in the top half and fix to the bottom part with M3 nuts bolts.