While Corsi boxes do work, I personally find that just using a single thicker 4" filter taped to the back of a fan works better, as it takes up a smaller footprint and allows you to direct the air flow at something other than the ceiling. The thicker filter is much, much less restrictive than a 1" filter due to the increased surface area. The thick ones (20x20x4") cost about $28 on ebay, and will last a very long time before needing to be replaced.