I got this fan for around 2 euros maximum and superglue for 1euro
I melted the plastic with a lighter (be carefull) i recommend wearing gloves (not the plastic ones)
Step 1: Steps
1)You unscrew the router.
2)Melt the top part around the mmeters you need.
3)Super glue the fan on it.
4)Screw back in.
5)Connect with 3-pin fan connector.