Better instructions without FilterBuy: https://cleanaircrew.org/box-fan-filters/

FilterBuy 'MERV 13' tested at MERV 7: https://www.youtube.com/watch?v=FKAVEek1YaSQ

TL;DR Filtrete 1900 performs best: https://www.bjs.com/product/filtrete-20-x-25-x-1-premium-all...

Also MIRV = Multiple Independently-targetable Reentry Vehicles, MERV = Minimum Efficiency Reporting Value. :)

DIY clean air is an awesome hack! Cheers