[schiffern]

Great tip. I should compare the cost-per-area across 4" filters too. Often I'm just making a Corsi Box[0], so the filter thickness is unconstrained.

Anyone got recommendations for a reputable vendor online? Something other than BezosMart?

[0] https://cleanaircrew.org/box-fan-filters/

[tempestn]

It might be a bit more challenging to make one of those with 4" filters, but yeah, worth a look at least. The other reason I like it is that our furnace is in the crawlspace, so having to get down there less often to change the filters is a pretty significant benefit. I'd do it even if the total cost were equal or higher.