Granted, I used a blower fan as instead of the box fan the canonical design uses.
Due to a series of unfortunate events, we've found ourselves with a reasonably clean basement. One of my wife's biggest complaints about the basement had always been dust. Dust from my very infrequently used wood-shop. Dust from the litter box. Dust from... well, dust.
The new basement seemed like a good excuse to take a crack at the dust situation and this design sprung into mind.
I built it with four 16x25 MERV 11 filters, some plywood, some 1x4s, some plumber's tape to hold the fan in place, and the aforementioned blower fan. I could have gone for a higher MERV rating, but I didn't feel the need to go super overboard. I can also crank the fan for multiple consecutive passes. That's the whole idea of the design after all.
The box itself is only held together with tape and the weight of the fan. Now that the basement is more together I might model and print some brackets to make the box more solid.
One cheap motion sensor plug later and the air down there is honestly pretty fresh
There's plenty of gap around the sides of the blower and pressure equilization will help even out flow, but you're right - the flow through all four won't be perfectly balanced. That shouldn't be a big deal and I can always rotate the filters if needed.