Review on 20x20x1 Reusable Furnace Filter for Home by Mike Larkin

These are smaller filters than regular 20x20 filters. When mounting to the ceiling, make sure the fan is running so it doesn't slip when the lid is closed. Unfortunately, the fan falls down when switched off and hits the lid. To overcome this I 3D printed a 10mm corner mount which I double sided glued to the corner of the filter. This caused enough compression for it to stay in place for the most part. I also 3D printed another 10mm wide spacer with a 5mm edge that goes all the way to the return cap to hold the filter firmly against the top and eliminate the whistling noise you inevitably hear. I don't know why they couldn't really make them 20x20 or even a little narrower. They give you flat screws if the hole is too tight. Judging by the reviews of many, there are probably not many cases when they are too tight. I can try covering all sides of the filter with a gasket for a better seal.