The cheapest way to make a filter is to order a MERVE13 filter from us (by donation) and duct tape it onto your box fan. Contact us: email@example.com. Filters are available on a first come first serve basis.AirFilterInfographic_FINAL
For more on why and how to build your own… https://pscleanair.gov/525/DIY-Air-Filter
Now Check this out!
The Corsi-Rosenthal box air filter.