>>106112593
Pretty much this, but you don't even have to go that far. Just attach one to the back with a piece of tape or elastic or something. Doesn't have to make a perfect seal, most of the air will have to pass through the filter regardless, and you can easily take it off to use the fan normally if you want. I've been doing this for years and judging by how the filters look when I replace them, it does a pretty good job.
Way cheaper than a meme air purifier that takes special filters that they'll stop making five years from now.