Objective

I wanted a fast print, a direct replacement of the stock duct and of course, better part cooling avoiding to blow on the nozzle and also something that wouldn't hit the clips holding my glass bed.

Another important thing was smoother inner airflow. All the mods I found had really sharp edges inside and I assume that would create a lot of turbulence and this fan is really week so that's a waste of energy. This one should perform a bit better in that regard.

You'll find three STLs here. The inner duct is exactly the same in all of them, it's the outer shell that changed. V3 is my favorite since it's the smallest and doesn't hit the clips on the edge of the bed!

All the mods I found were either too bulky with very little benefits or didn't fit correctly. Another problem I found was that similar mods were also a bit large and they would hit the clips holding the bed glass.

I measured the distance to the nozzle and the center of the curve of the exit part of the duct should more-or-less be there.

Just keep in mind that there was a lot measurements by eye and it's not something designed by an engineer :P

I did do some water tests and they're ok I guess. Definitely better than the stock fan duct.
I would like to make the air blow a bit more under the nozzle since right now I think it's a bit far, but still, works nice.

Results

I did find noticeable improvements in overhangs, doing a 70º overhang test I don't see as much curling and the bottom face is much smoother. Still, not something too huge.

Design

I made this in Onshape, if you want you can copy my design and modify it: https://cad.onshape.com/documents/ace39ddaf1738f8b2d2022f4/w/6b3fa3041e016c1bdb6e6c2f/e/f9009f8c18746042c4282d8d