I built a filter box, an evolution of the Comparetto Cube/ Corsi- Roesenthal box ( no idea who designed it first).

My evolution I am calling "the Poindexter." Like many other wood stove owners I have a 20" box fan like this running on low in the hallway of my ranch style floorplan to gently waft cool air from the floor of the bedrooms down the hall towards the stove, sort of enabling warmer air from the stove to waft gently along the hallway ceiling. I end up with less temperature differential between the back bedroom and the stove room with the fan running.

Here is the prototype, 1 fan, three filters, the remaining two sides of the cube are the cardboard front and back of the box the fan came in. Duct tape and sharp knife. No measuring, just tape it up and cut the cardboard to fit during assembly.

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It 'seems' to be working, I have two air quality meters on order, but there are two problems. One, it is not very rugged at all. Two, it would take up less space in the hallway if I could lift it off the floor a little bit so I have a filter on the bottom surface, and a blank (cardboard, plywood) on the left side as pictured, then scooch it up against the wall to the left. A plywood top would be good, I have found our cat napping on top of the pictured item twice already.

So I have a plan. If I start with a 48x48 sheet of plywood I can make a floor big enough for the box fan to sit on, and the top, and a front upright between the top and bottom with a circle cutout the size of the fan blades, and a 20x20 blank to go on the left side that could be moved to the right side if I want to put it on the other side of the hallway.

My thinking is to stack 1x2 furring strip in this shape (looking from above) for the columns at the back:

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So looking at those from the side there would be a face frame to duct tape a furnace filter to, and a resting surface for the back side of the filter to rest on.

My current thinking is I could make an X brace between floor and ceiling at the back with more furring strip and that ought to be enough. It will cost me a tiny bit of airflow from the filter on the back wall, but I am also thinking it may not be enough. I want to be able to mount a handle near the center of gravity so I can carry it up and down stairs as needed.

Thanks.