If you don't mind playing around with a Raspberry Pi and a few jumper cable, have a look at my repo. Half a year ago, I built a fan controller to regulate temperature and air flow in my server cabinet with 5V PWM fans. It is not finished and rough but it works.
After moving in a few months, I will have the time to work more on it.
Fan & thermostat for cabinet cooling
I want to put some devices like NVR, modem, router, et cetera in a closet....