I wanted to mount a 4" exhaust duct through a casement window to vent ABS fumes from my printer, so I made this backing plate for https://www.printables.com/model/474337-120mm-fan-to-4-inch-duct-f3d-file-for-other-sizes so that I could bolt it through a flat surface (with no 120mm fan at all - I already have an in-line duct fan).

If interested, you can see the full setup at https://www.printables.com/model/525118-coupler-for-4-duct - I didn't post the final picture here in this design because you can't actually see the backing plate in the picture. :-)