Yeah but there are better box fan filters out there. You can do exactly what OP did with his cube and throw a box fan on top of it instead of the duct fan, and

Yeah but there are better box fan filters out there. You can do exactly what OP did with his cube and throw a box fan on top of it instead of the duct fan, and with the higher airflow from the box fan combined with the exact same filter setup you'd likely see better results. Most of the box fan filters I've seen these days look like this cuboid fan- as in, they have four filters, a base, and a fan. Many also include some cardboard curved corners to improve airflow and reduce noise. I kind of assumed that's what we were comparing until I saw the image. Like these from another thread: https://cleanaircrew.org/box-fan-filters/ https://cleanaircrew.org/box-fan-filters/