I built my 1st box a few days ago.
I used 20" X 20" Merv 13 BNX filters sourced from Amazon.ca and a house branded box fan from HomeHardware.ca.
I learned I could have used taller filters(20"x24", 20"x30") and maintained strength and stability.
After a bit of experimentation I arrived at a fan shroud opening of 16". This eliminates any noticeable back flow around the fan perimeter.

