[marcinzm]

This may or may not work, cheap box fans aren't designed to push/pull air through obstructions (ie: low static pressure) and HEPA filters are giant obstructions. So instead of the air going through the filter it'll go through gaps, around the edges of the blades, etc.

I used a cheap box fan for some DIY ventilation and the loss in airflow is massive after filter and ducting.

[jakeogh]

I have a box fan on low 24x7 with a 12 on the input and a 7 on the output. That's not a HEPA rated filter set, but it's fantastic. Allergy season just went away (while in my room) when I started using it a few years ago. I also tried a 3 on the output, and that worked as well, but the flow was noticeably lower. The fan is cheap, $20. I change the filters every few months, and it always looks like I waited too long.

[UnFleshedOne]

I used a cheap box fan and a furnace filter held simply by pressure differential.

Flow drops a lot, but it still works. Filter was dirty after some time. It was hilariously undersized for my purpose (serious belt-sander action leveling sub-floors), but air does move.

[Spooks]

couldn't you easily test this by sticking your hand (wet it first) in the middle section of the filter and see if you feel any air coming through

[dunkelan]

You could, but I think the goal would be to make sure the design will work before going through the effort of making it. If you already have all the parts though definitely worth a try.