Update 18.09.19. I no longer own the Prusa printer so my ability to support this mod is limited.
This is a very simple and elegant strain relief for the X-axis motor cable on a MK3. It closes around the motor and the cable and is then tightened with a zip-tie. Inside the cable grip it has small teeth that hold the cable in place.
Please be aware that it is meant to be used only with the textile sleeve. It will not work with the older spiral wrap.
If your Prusa came with spiral wraps, you can purchase textile sleeves yourself. The brand Prusa is now supplying is HellermannTyton, and the sleeve diameter for the X-axis motor is 5mm. The part number is 170-01000 and you can purchase it from Conrad if you live in Europe. More information is on PrusaOwners Wiki.
The part has been verified to fit by several different people during beta testing, but because it depends on two close interference fits, it may still be too tight or too loose for you. I provide the Fusion360 file where you can adjust the inner size of the collar or the inner diameter of the cable outlet, depending on what needs adjustment. You will find the required dimensions in the model parameters.
The correct orientation is such that the cable is directed away from the printer by the outlet.
Please use common sense when installing, you need to tighten the zip-tie just enough so that you can't easily pull the cable sleeve out or displace the collar on the motor body. Holding the eyelet of the zip-tie with one hand and pulling the other end of the zip-tie with pliers will work, just don't use gorilla force.
Credits to @stahlfabrik, for his valuable feedback during design and photo of the installed strain relief, as well as to "602 Wasteland" Discord group for their support and motivation.
Don't print in PLA, as the motor may get very hot and the part may soften and fall off.