Which Cowan units do you use? The only concern I have with smaller air filter devices is they sometimes can have significantly less surface area than the furnace filters typically used with box fans.
The Airmega 400 for the main living room and a pair of AW-1512/Airmega 200 (they're the exact same unit with cosmetic differences, get whichever you can find in stock unless you strongly prefer the look of one). Surface area wasn't my main focus. I'd rather change filters more often (not that that's a huge issue outside of wildfire season) and deal with a quieter unit.