I found this Idea on Chep’s/Elproducts youtube channel and he got it from OneBadMarine.
https://www.youtube.com/watch?v=Fb4XMbZ0iA4&feature=youtu.be

It wasn’t possible for me to slice Cheps models in SLic3r so I recreated it in Fusion 360 and then changed it quite a bit to fit Ender 2 and 3 better and improve performance. I tested this for a few prints and it works well.

Changes:
5mm longer fitting.
Deeper cuts in the fitting (more flexible when compressed and less “angle”)
Inside dia decreased to 4.1mm (from 4.2) and added a cylindrical taper outward (so that the gripping surfaces are parallel when compressed).
Thread changed from M10x1 to M10x0.9 with 0.03mm negative offset (slightly smaller tread)
Different position for the collar.
Flat surface for the collar so it handle the force better
Nut inside diameter decreased 0.4mm
Cut out in the nut for the screw head on Ender printers.

Tube length:
Cut the tube so that it is level with the top side of the cooling fin block, in my case 38.4mm.

Tube compression:
The collar is located 1mm below the topside of the cooling fin block so the tube is compressed 1mm when the fitting is tightened. My tube was 38.4mm and compression to 37.4 requires a force of 130N~39LBs. You can make the tube 0-1mm below the cooling fin block if you want a lower force.
When a 4x2x38.4mm tube is compressed 1mm lengthwise it will change shape to 4.05x2.02x37.4. The inside diameter will theoretically increase which is a little counter intuitive. The bottom will probably change size more because of higher temperature.

Only use your fingers when you tighten the nut or you risk to compress the bowden tube too much. Check with a piece of filament. I’m not sure this will work well if you have a 1.9mm tube. My new Capricorn XS tube is definitely bigger the 1.98mm, probably 2.05 (specified to 1.9+-0.05) so don’t count on Capricorn to keep their tolerances. Capricorn Dy was 4.00 and Creality 4.1. I made this for outside dia 4.0mm

Stl, step and Fusion files included. Note that M10x0.9 is not included with fusion 360 so you need to move “ISOMetricprofile CUSTOM.xml” to “%localappdata%\Autodesk\webdeploy\Production\Fusion\Server\Fusion\Configuration\ThreadData”

Update 2020-01-15:
Make sure the hole in the coupling has the same size or little bigger than the bowed tube otherwise will it be difficult to do an atomic pull since the end of the filament will have the same size as the inside if the short tube inside the throat/heatsink. Dia 1.9 tube inside the throat/heatsink and 2.0 in the Bowden tube is probably optimal.

Update 2020-10-26 (version 3):
Barbed design for better grip.
More flexible “blades” for even pressure. It is also easier to verify that the Bowden tube is pushed in all the way.
Through hole version for extruder added. Can be used when extruder fitting has been pushed out of yout BMG and you have a M10x0.9 tap.
Nut inside dimension increased.
Material changed from PLA to ABS (PLA should still work)
Note that Thingiverse refuse to update the STL pictures for some reason.

Update 2020-12-10 (version 3.1):
Compression was only 0.5mm for some unknown reason. Increased to 1.0mm
Removed “dummy” part (it was only for tube length verification anyway).

Print direction:
Coupling is designed to be printed with M10x0.9 thread up. Raft with no support.

Creality

Ender 2

Yes

No

0.2

30

Coupling
Layer height: 0.15
4 walls
Raft: Yes

Nut:
Layer height: 0.2
4 walls
Raft: No

Dummy
h 0.1
4 peri
3 top bottom
Raft: No