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

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.