I’ve placed a 120 mm fan on (near) one side (because the holes are on the sides). I recovered the fan from a dead power supply and bought a fan to USB small cable adapter. It would work with a smaller fan but those large ones are probably quieter. Then I placed the router above my CISCO SG250-08 switch that also has holes ont its sides. That’s one fan cooling 2 devices at the same time.
Oh, it looks like a very smart choice at first glance.
I bought a 92mm case fan from my favorite Scythe and diverted it, but it was a waste of time to prepare a fan guard and a 12V input
MV-1000W which used to generate heat up to 60-80 °C, no longer exceeds room temperature (25 °C, CPU temperature is about 45 °C） after the fan is installed on the bottom.
Therefore, it is not necessary to consider the possibility of insufficient cooling performance after adding an air cooling fan. On the contrary, the fan can be rotated at a slower speed to reduce noise.
Those who have some knowledge of board handling should be able to script control PMW via UART or USB. Thanks to @cafebug for telling me that CPU temperature can be easily obtained!
Even if this is not the case, many 12V fans will rotate at low speeds by applying a voltage below that.
Alternatively, a simple PMW fan controller with an integrated temperature sensor seems to be available for under $5.
That said I didn’t really care about the noise, and it was troublesome to handle the pins, so I always keep turning at 12V (2300RPM)