Tools/materials used:

M10x1.25mm tap (to cut threads, optional)

10mm open-ended wrench

Silicone grease

1 ea. PC4-M10 straight pneumatic push fitting connector

2 M3x8mm hex screws (to mount device to printer back)

87.5 mm length of 4 mm (OD) / 2 mm (ID) PTFE Bowden tube (see assembly notes)

Design overview

I wanted the ability to easily switch over between filament sources (side mount, back mount, or one of a couple of drying boxes) and noticed that I'd have to fiddle around, often removing the Bowden tube, when feeding new filament in past the filament-out sensor because the filament wasn't aligned just-so. When I came across this "Upgraded side spoolholder for the K1C with filament guide" by @FransKV (https://www.crealitycloud.com/model-detail/upgraded-side-spoolholder-k1c-filament-guide?source=5) design, the guide-part of the design seemed to address the issue I was having because it kept the filament in the proper up-down orientation when feeding into the filament-out sensor. However, the legacy design was made to accept filament from one source (the side-mount spool). This design adds a PC4-M10 straight pneumatic push fitting connector to the concept to allow swapping filament sources.

Assembly

Once printed, temporarily mount the guide to the back of your printer (below the filament-out sensor) using two M3x8 mm screws. Thread a piece of 4 mm (OD) PTFE tubing through the guide until it seats in the sensor fitting. The first time you do this, you may find a snug fit, but it should work. Cut the tube at a right angle just below the bottom guide hole (where the push-fitting will screw in). (For reference, the length of tubing that I got from this step was 87.5 mm long.)

Now, remove the guide from the printer and remove the tube from the guide. It's time to cut the threads for the push-fitting connector. With patience, can use a wrench and the fitting to do the job (just be sure to work the fitting back and forth, and go slow). You may find that applying some lubricant, like silicone grease, to the fitting threads (after the first few threads are cut) will make things progress more smoothly. Thread it all the way to the shoulder of the connector. The task is easier if you have an M10x1.25 mm tap, though. Grease the threads of the fitting with silicone grease to make threading it on easy.

With the connector installed, reinstall the tubing and mount the guide assembly back on the printer. Put a short length of Bowden tube on the bottom connector and guide some filament in past the sensor, ensuring that it travels smoothly through the assembly.

That's it. You're ready to rock.

75mm K1C Filament Guide with M10x4mm Bowden Connector

0.2mm layer, 5 walls, 50% infill