Edit 2019-06-12: Added a new version that directs the airflow to the sides. This might Prevent warping as the stepper cooler doesn’t blow the air on the printing bed. No suppurts needed. Prevents the extruder stepper from getting too hot. I noticed that specially if printing with high bed temperatures, the extruder stepper gets very hot. The stepper might be made to take such temperatures but I just like things to be nice an cool. I secured it with CA-glue on the stepper. There are two surfaces facing the stepper on which you can apply glue or double sided sticky tape. You can take power from the existing extruder cooling fan or as I made it: Build a 24v to 12v step down converter into the EINSY-RAMBO case and route a new 12v cable to the extruder. Edit: Version 2 uploadet. It fits the Nema17 Stepper better now.

Source: Prusa i3 MK3 Extruder Stepper cooler by CG-Tech