I have an air inlet for my HVAC system in a closet, and that closet gets very dusty. This thing allowed me to mount a 1" HVAC air filter on the vent grill intake on the inside of the closet. (without installing an hvac duct which would fill the entire closet)
it has screw holes so you can fix it to the wall and a pass-thru hole on the front side so its easy to do so.
you could print 4 and fix at all corners, but i was able to use just two.