The original fan design on the GEEETech Pro C seems a bit useless. I think it would be better to move the air flow under the steppers so that the nozzles, as well as the filament that had just been extruded, could cool better and quicker.
This design is a remix of the STEV-o's fan holder and was adapted to the Pro C with MeshMixer. I'm not a 3D designer, however it will print nicely without any support.
To install it, please unscrew the fan from its original location, then screw the holder on the fan support. Finally, screw back the fan on the holder. Smaller screws will be needed since the holder is thinner than the fan.
The fan can be unplugged from its original connector and plugged on the FAN_PWM connector. That way it will be used only when printing (or controlled manually). Don't forget to configure your slicer to use it when printing.