This is a guard to keep fingers and filament out of the fan I will place on my extruder. I found that the fan only had a guard on the outlet side facing the extruder and not on the inlet side facing the rest of the world, so I made one. Print the guard (it takes only a few minutes and 14 cm of 3 mm filament) and attach it with two or more screws through the fan and into the fan mount of your choosing. The holes should fit the standard holes in most 40 mm fans.