I have found that the stronger plastic boxes can accept the m6 threads of the pneumatic couplings we all seem to use. But just incase I used a wide printed nut... I found the coupler would scratch the filament as it entered.

So, I made the nut deeper and added a fillet to help guide filament directly through to the coupler.

v2 nut is bigger and deeper to allow a smoother guide as the filament unwinds from max radius to min radius.

Freecad file also available

Geeetech i3 prusa pro w

No

Doesn't Matter

0.3mm

15%

I use taps to cut the required threads. Or get a m6 bolt and cut 2 lines down the threads, perhaps chamfer the bolt for an easy start.

This will serve as a crude tap.

The FreeCAD file have been added.

There are other better solutions, that are more involved. This requires 1 print, 1 box, 1 coupler, 1 hole and perhaps a tap to cut the threads..