Used and modified the part from tsimon57 (thanks man!).
I think the last version (v7) is the best, because there is enough space to fit on the extruder cover and you can screw the fan on the front. The versions before are also very difficult to print. It was a lot of support necessary. For the v7 you only need some support on the "hooks"...
Main goal was to cool the extruder gear, because the heat goes up from the heater to the gear, and after a while of printing the filaments gets to soft within the "gearbox" and the extruder stops extruding...
6mm thickness (measured it and it is 7mm)
I soldered it to an USB plug, because I use a raspberry pi with octoprint and M33-Fio, so the fan is powered when the OctoPi is online.