It took me about 50 minutes to build this Corsi-Rosenthal air filter box with parts from Home Depot here in Toronto. It's a 20" box fan, four 20"x20"x1" 3M 1900 air filters (1" deep), some duct tape and cardboard. The cardboard came from the fan's box and from a grocery delivery, so you can use just about cardboard you've got around. The shroud has a 15" diameter hole, as suggested by David Elfstrom on Twitter; he ran some tests on this particular fan model and so if you have a different model you may wish to consider a different diameter.
Many thanks to Jim Rosenthal ( / jimrosenthal4 ) and David Elfstrom ( / davidelfstrom ) for all their efforts to keep us safe and for their specific feedback on this attempt.
References:
* PDF plan: edgecollective.io/airbox/
* Article: www.wired.com/story/could-a-j...
* Article: www.wgbh.org/news/local-news/...