In my area, we have annual fires that get pretty bad. Add in ag related pollution, allergies, etc…and air quality is almost always unhealthy.

Our local makerspace has partnered up with our local asthma collaborative to make custom PM sensors here: https://www.sjvair.com/

We have tried out a couple of designs and had actual science done on which custom air purifiers did the best. One of the members rigged up a airtight area and pushed smoke in. Got a couple of readings, then did the same test with the air purifier designs.
We found a couple that work well.

https://github.com/opennukit/Nukit-Open-Air-Purifier/ is a good one.

One that we improvised was the following:

RzRQd8q8LGgXGow.jpg

jC0NifJrItD5nvE.jpg

3d printed brackets, a filter, and some activated carbon and you have a good air purifier. Family uses these for white noise and air purification at night. It works wonders! The fan is 20$ at any wallmart, the filters are dirt cheap. And you can get huge rolls of activated carbon on amazon. No need to over complicate things, just bracket everything on, let it sit for a good 3-4 months, then look and see if anything needs replacing.

One note of caution, some of the huge box designs like the one OP is suggesting was having trouble with the engine overheating. Most of these box fans use the air to cool off the internals. If it doesn’t get that for many hours, it can start overheating. Just check after running whatever solution you come up with and make sure the engine isn’t getting hot.