The duct fan + filter combo is how lots of cylindrical consumer units work (e.g. Levoit, Philips, etc). They just use crappy fans which don't really pull any ai

The duct fan + filter combo is how lots of cylindrical consumer units work (e.g. Levoit, Philips, etc). They just use crappy fans which don't really pull any air through, or they're very expensive for what they are. In terms of availability, I think the current popularity helped, otherwise you could also look at the auto market, lots of cylindrical filters there and they'd be fine for pulling out stuff like pollen, though you might want a carbon filter as well. I have no idea what's required for HEPA certification, but lots claim to be: https://www.amazon.com/LEVOIT-Core-Replacement-High-Efficiency-300-RF/dp/B07RSZSYNC https://www.amazon.com/LEVOIT-Core-Replacement-High-Efficien... It has an outer dust mesh, the HEPA filter and a carbon insert. They're all the same though, I think you could use any manufacturer's filter. Longevity isn't really an issue if a supplier goes bust, just re-make the adapter ring. Pretty much all you need and you don't need to worry about sealing the corners. According to OP's link, they use a duct fan booster which is a lot quieter than a box fan (at least the claim is it can go down to 16 dB). Though a question is does a can-shape system work better? Or would you get the same effect if you used a single flat filter with the same "unrolled" area? I have no idea, and there's so much uncertainty involved with DIY testing.