I call this an "Amish" air purifier because I learned it from a coworker who claims to have learned it from an Amish person.

Of course, this isn't very Amish in design as it requires electricity and English manufactured goods. Also a 3d printer.

Maybe there's a better name for this type of air purifier because apparently it's not a novel idea...

Ah, I utilized my god-like power of access to a search engine to learn that this type of purifier is known as a "Corsi–Rosenthal Box"

Of course, you'll need to buy 4 HVAC style air filters for this project. The ones I bought are 12"x24"x1" because that was the only size my local Home Depot had in stock that were FPR 12/MERV 13. The higher the MERV number, the higher the filtration, and for this use case, anything lower than MERV 13 just won't be super effective. Side note, my understanding is that you actually shouldn't use a filter with such a high rating in your HVAC system because it will cause extra strain on the motor(s).

This bracket should be compatible with any 1" wide filters.

Print a minimum of 4, maximum of 8, one for each corner, top and bottom. Put all the air filters together in a square. Make sure to put two of them close enough together so the fan may rest on the edges. If your filters are bigger than your fan like in my setup, then you'll have a gap that you need to cover with some piece of scrap, like a box or spare plank.

I feel silly for not thinking about placing them in the arrangement shown in the pictures I just uploaded.

It's such a simple model I don't feel the need to include many printing notes. I used and would use PLA for the sake of cost and since this model isn't designed to bear any weight, heat, or chemicals. You can also print with like .25 or .28mm layers, or even bigger if you use >.4mm nozzle.