How good is to tape a hepa filter to a box fan? I think I recall a marketplace episode where a scientific or whatever said that it was better than some shitty products and good enough for the small price.
This is mitigated by using four filters - constructing a frame that holds them top and sides - and the box fan goes on the bottom, with legs to hold it off the floor.
Box fans are also very noisy. The box-fan design was intended mostly for people in wildfire areas who needed something to mitigate smoke inside their homes.
Probably minimal impact on other pollutants (VOC's, NOX, CO, etc)... Many professional air purifiers don't make any claims about those either tho.
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.
The main downsides are comparatively ugly and noisy.