Furnace filters are designed to work on the intake / vaccum side of the furnace fan. All the successful improvised designs I've seen place the filters on the intake side of the box fan; multiple filters arranged as a box there increase the airflow greatly.
Years ago I tried something similar on the exhaust / pressure side of a fan. Total fail.
Edited by dougwalkabout (06/11/23 02:34 AM)