If you want to DIY these there's a FOSS (GPLv3) parametric builder web app that will output files for 3D printing, lasercutting, or handcut foamcore and calculate the performance of different configurations for you:
There's no real performance difference between DIY and metal PC fan filter boxes, just durability.
Youden 3 hours ago [-]
A YouTuber, Joel Creates, recently developed a filter that can be assembled from furnace filters, 3D printed parts, O-rings and PC fans: https://youtu.be/GpA7-EJpKWU
https://www.filterboxbuilder.com
There's no real performance difference between DIY and metal PC fan filter boxes, just durability.
Files can be purchased by backing the Kickstarter but they won't be sent until November: https://www.kickstarter.com/projects/joelcreates/oddcube-air...
I think this could be substantially cheaper than a build based on a PC case.
About the same as a Coway
https://housefresh.com/coway-airmega-250s-review/
https://housefresh.com/nukit-tempest-review/