I made these for 1) for a post construction clean-up project - lost of dust getting kicked around. Worked like a charm 2) wildfire smoke - Happened to have the material from 1, & it did seem to help

The trick is that the box fan turns over the air much faster than a conventional hepa.

https://aghealth.ucdavis.edu/news/corsi-rosenthal-box-diy-bo...

Still, $10 for the ikea filter seems like a good deal if you get good life out of it. 20x20x2 at filterbuy is about $16.5 x4.