Derived from Owen's fan duct design. I liked the clip-on design, but after a few high-speed prints, it started moving around. I added "ears" that allows screws to be added to securely clamp the duct to the threaded rods as well as allow you to adjust the height.
I added a rib to the interior of the duct to increase rigidity where it was a little weak due to the how it is printed. I also added some "flats" to hold the hex nuts in position to make assembly easier. I also removed a bit of material "here and there" to lighten it up a bit.
Owen published his source Autodesk Inventor CAD file which is a great design - Thanks! The modified Inventor and STEP files are included as well.
0.4 wall, 100% fill.
I printed at 75 mm/s, 0.1 mm layer height.