I built a window vent with a variable-speed computer fan and an esp8266 that communicates with a CO2 monitor and raises the speed when the CO2 gets too high. I have a very fine mesh on the window screen that's designed to filter out pollen and dust. It works really well. I usually only turn it on at night, because that's when my wife and I are both in the room and the CO2 can really build up.

I use an ikea air filter that just so happens to fit perfectly in the window + a smart outlet and co2 sensors.

I’d like to do a custom build like you so the fan speeds are variable.