Some countries don’t have box fans, but have round fans.
“Presenting V1.0 accessible, effective and validated DIY filters using round fans inspired by the #CorsiRosenthalBox. They are the result of international collaboration from designers, engineers and advocates in @TheWHN Airborne Group DIY Filter Team.”. Source here.
For the type of fan, “For gym fans, anything between 80-120w 16-18″ is best. Aim lower for quietest operation. These designs and numbers were tested using a 18″ 45 cm gym cage fan. Best not to go above 120w. Will likely be running setting 1 or 2. Highest power flow is large but unstable & noisy.”. Source.
“Calculated based on 62% filter efficiency but we have particle measurements for Aerostar brand. We’re building and testing in UK,Spain,Canada,Switzerland. “. Source.
4 Filter Build
1 Filter Build
1 Filter Build Video
2 Filter Build (Wedge Shape)
Some creative ideas:
4/ 5 Filter Build
A most excellent idea is this one, by ChurchAndCoVid : “The plywood (OSB) is 21”x21” (53 cm). Fan diameter is ~16” (40 cm) This could go on top of 4 merv-13 filters or a frame with polypropylene sheet.”
A round fan that might work with a box combination:
A round fan video build below, found by David Elfstrom: