it would be interesting to measure the airflow with a decent anenometer that works at low airspeeds (I've had success with https://www.amazon.com/BTMETER-Handheld-Anemometer-Velocity-...).

My intuition is that the 5 case fans at 72 CFM each are deeply underpowered for the job of moving air through a filter medium, especially in an array pointed into a mutually turbulent unsealed box, and that you would do way better with something like an AC Infinity Cloudline S8, which moves around 800 CFM, aimed directly at the filter in a sealed environment.

It's pretty common for case fans (and even industrial fans) to be used for cheap airbrush stations, which have much smaller filter face openings. These generally don't pull even 1 linear feet/min at the station face, even though they're theoretically rated for over 150 lfpm without the filter media in place.

There are actually two different types of case fans - high airflow vs. high static pressure.

The CFM number alone doesn't show that.

I assume the high static pressure ones would do a much better job in this context.