Just built this quick project from an article in “Woodsmith” magazine. I made a few changes from the article.
There were incorrect measurements in the exploded drawing in the magazine. For example, the top piece was labeled 21.5 inches, when it clearly needed to be 20.5 inches, like the bottom piece.
When I determined that some measurements where reported incorrectly, I gave up on reading the plan in the magazine and I started making some changes to the design. I based the size of the inner components of the wooden frame on the size of the filters.
I also decided to remove the inner fan grid and screw the fan housing to my wood housing. It makes it solid and keeps the fan from falling out.
You slide two 20×20x1 furnace filters in the slot in front of the fan.
I used a fiberglass filter in front of a pleated filter. The pleated filter is not rated as high as I would have liked; there was a filter that claimed micron level filtering. It significantly more expensive than the pleated one I got. I wanted to see how the box fan motor handled the pleated filter to begin with before I spent more on the micron level one.
So far it seems to work pretty well. It is very portable, and the wooden frame makes the fan very strong. The frame also makes the fan extremely stable.