Not sure about the bearings/sleeve liking that orientation, but 4 20x20x2 filters will make for near-nihil impedance to airflow. (Which means more CFM, meaning higher AER, meaning less (arrestable) particulate in the air.)
I'll try that design out this wildfire season.
(Maybe even 5 filters, and 'legs' to elevate the unit)
edit:
Home Depot has 12-Pack Rheem Household Pleated High Performance Air Filters (MERV 8 - FPR 4) on sale as listed below for $19.97. Shipping is free. Thanks to community members tunabreath and spa ...
slickdeals.net
~$20 for 12 MERV 8 filters.
-not
great for smoke, but will work to greatly reduce dust (as OP was interested in).
12x of those makes 3x quad-filter box fan units (as
@mechtech 's video showed, but 1-inch thick filters.)