Firstly, I used the connector end from PTFE Bowden Tube Connector, so credits to @firstgizmo for that portion. The main aspects of this, however, are completely unique.

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This inline filament buffer was designed out of the desire to use retract before cutting on the K2 Plus with CFS. When we first tested this idea, we found that the retraction caused a great deal of pressure in the PTFE line, often causing it to pop out of the extruder, requiring user intervention to correct. Additionally, that connector is failing prematurely for many users, so we do not want to put any additional stress on it.

This version of the inline filament buffer will allow ~20 mm of travel with minimal pressure on the line. Presently, I am limited by the length of spring that I have, but I will be making another version for a longer spring once I get those springs.

Here is a slice view of the buffer

The filament enters from the right side, extruder is on the left side. When the extruder retracts, it will compress the spring , thus lengthening the overall line distance.

You will need a spring

I am using a 0.5*6*25 mm spring from https://amzn.to/40D6JnN. 

You will also need 4x M3x10 screws such as

https://amzn.to/4hYaX0k

https://amzn.to/4hWkp4n

https://amzn.to/3EDaQZO

Right now, I'm not happy with the friction connection of the PTFE and the slide, but it is working well enough. I will in the future try to find a better friction connection there.

You can then edit your machine g-code in your slicer for the change filament G-code section to the following

G2 Z{z_after_toolchange + 0.4} I0.86 J0.86 P1 F10000 ; spiral lift a little from second lift
G1 X0 Y245 F30000
{if toolchange_count > 1}
G1 E-20 F200
BOX_MOVE_TO_CUT
G1 E25 F200
G1 E-28 F200
{endif}
G1 Z{z_after_toolchange} F600

This will do the following

1. Perform spiral lift
2. Move to cut position
3. Retract 20 mm
4. Extruder 25 mm to push the cut tip back down in the nozzle
5. Retract 28 mm to make sure the filament tip is clear of the cutter
6. Then it will proceed with the filament change.