If a single filter is used, the fan might fail. Using additional filters decreases the load on the fan, because there are more ways for the air to come in, so the torque required for a certain CFM value (which stays fixed for a certain fan speed) is lower.

I saw that pic and immediately wondered why they didn't seal off the areas of the fan that the filters didn't cover. I'm willing to bet that had a negative effect on the box fan setup's performance.

I'll take that bet! https://dynomight.net/2020/12/15/some-real-data-on-a-DIY-box...

I couldn’t find a way to contact you—could you specify the filters you used for your cuboid design?

I found this strange, too. They don't actually cover the fan as well as the intake to the cuboid. I think comparing to the single large filter would be more honest.

The fan will burn out if you use a single filter. See my other comment in the thread.

Can't argue with that, I've got two burnt out box fans ;)