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Here's the 20" #CorsiRosenthalBox I'm working on. Fans in rectangular housings basically don't exist in the UK, so I made a cardboard shim out of the box for this circular one.

This build was a proof of concept (hence the green masking tape rather than duct tape, and the plastic sheeting still on the #MERV13 filter boards). I was testing whether the shim and the boards would support the weight of the fan. Despite being a mock up it's surprisingly rigid!

Next stage involves building it properly (including sealing those gaps at the corners; the 20" fan is actually more like 21.5" across and the boards are 19.5"), then using my anemometer to measure the flow rate of the finished box, so I can work out what size room it can filter. I'm intending this one for a local community centre where a Queer Club meets, to make their meetings safer.