Having built several of these for friends and family along with a lot of reading on these and air filtration in general, there’s a few things I want to bring up.

First: scoff all you want that this goofy looking thing was designed by “experts”, but the design choices they made resulted in something that’s cheap, fast to build, and works exceptionally well. They knew what they were doing, and there’s good reasoning behind the choices they made.

You need to use MERV 13 rated filters, and ideally at least 4 of them.

Air filters have ratings, the common one in the USA being MERV, explained here on the EPA website.. Short version: bigger number catches more (and smaller) particles, but is more restrictive and expensive.

MERV 13 is a good compromise, as each time air passes through it will capture particles:

• 0.30-1.0µm greater than or equal to 50%
• 1.0-3.0µm greater than or equal to 85%
• 3.0-10.0µm greater than or equal to 90%

Far cheaper than HEPA (MERV 17) and commonly available in a size that works well for this setup. With 4 or more of them a box fan has sufficient airflow to recirculate the air and capture even more particles as time passes. You can use 1 filter, but the consequence is greatly reduced airflow as MERV 13 is fairly restrictive. The better the airflow, the more effective the system is.

The “shroud” (cardboard with a hole for the fan) is essential. It prevents unfiltered air from bypassing the filters and creates higher negative pressure which increases flow through the filters. It also makes the whole thing noticeably quieter. The actual experts have already figured out the best size hole for a couple popular box fan models so you don’t have to experiment with what works best.

As for the fact it’s taped together … it was designed during COVID to be cheap and fast to build. You can absolutely make improvements to the design – I’m doing so myself. But I also built one per the directions using painter’s tape so it’s easy to tear down once I’ve built a proper enclosure.