Great advice, but build a Corsi Rosenthal box instead as it will increase air flow to the fan, keep the air cleaner and make your box fan last longer.

https://engineering.ucdavis.edu/news/science-action-how-build-corsi-rosenthal-box

https://www.cdph.ca.gov/Programs/CCDPHP/DEODC/EHLB/AQS/Pages/Corsi-Rosenthal-Portable-Air-Cleaner.aspx

and here is the case study around it:

https://energy.ucdavis.edu/wp-content/uploads/Case-Study_DIY-Portable-Air-Cleaners-083121.pdf

Now, where to buy cheap and reliable quality filters?