I use 20" box fans for supplemental air cleaning in my shop. This is a common practice, but the way to attach the filters differs greatly. I wanted something that would attach to the fan using the existing screws and would be small/print quickly.

This was relatively quick to design and print. You can check out the actual Fusion 360 file at http://a360.co/2BAdHhv or review the STEP file attached.

Ultimaker

Ultimaker 2

Doesn't Matter

Yes

0.35mm with a 0.6mm Nozzle

30%+

I wanted this to print fast, so I selected a larger layer height with a larger nozzle using old, unnamed PLA @ 220° C with a 60° C bed. My infill was very low as there were very few infill layers at .35mm layer height. If you were using a finer pitch, I'd recommend 30% or greater infill.

Using Cura I rotated the piece to lay flat on its back (rotated 90° and used the "Lay Flat" function.) If your slicer doesn't have that function, you can rotated by 95.36°

The part was designed in Fusion 360 (http://a360.co/2BAdHhv). I used a parameters for many of the settings, so feel free to open it up and see how changing the parameters changes the design.

I sketched the base first, including the screw hole and counter sink. I extruded the base portion to {baseThickness}, leaving only the rear that was eventually lofted untouched.

Based on the original Sketch plane, I created an offset plane that was offset by {baseThickness}*2 + {filterThickness}. I started a sketch and projected most of the base sketch pieces onto it. I made some additions that would allow the standard to slant inwards. From there I lofted the rear of the base up to what will end up the top of the retaining piece. I then extruded the rest of the retaining piece down by {baseThickness}, allowing the rear surface to remain flat.

From there I applied a gratuitous number of fillets.