There have been several submissions of (various? or the same?) DIY air filters.
The trick to finding them on hn.algolia.com appears to be to use 'air purifier' rather 'air filter' (which kind of suggests that they're submissions about the same one):
Or using Google site search (the trick here being to search for "points by" to limit results to the toplevel submissions (and incidentally submissions with not exactly 1 point :)):
https://www.google.com/search?q=site%3Anews.ycombinator.com+... - «site:news.ycombinator.com fan filter "points by"»