Comment by JohnicBoom

You can use more than one filter. I've heard it called a Corsi box: https://www.texairfilters.com/a-variation-on-the-box-fan-wit...

Basically, you're making a cube. One side is the box fan, with the intake coming from the inside of the cube. The other sides are taped together filters. It significantly reduces the strain on the motor of the fan, and you can use the cube for a long time before replacing the filters, since each one is filtering so much less air.