Model originally uploaded to Thingiverse at https://www.thingiverse.com/thing:4093675.

This is simply the bullseye duct slightly elongated so that it will fit a 40mm x 40mm x 20mm fan (e.g. the noctua). As standard it will only really fit a 10mm fan without blocking air flow. The 20mm fan pushes more air than a 10mm fan, allowing a quiet noctua fan to be installed without dropping airflow too much.

Please note, I take no credit for the design of the duct. All credit goes to the original designer which can be found here...

https://www.thingiverse.com/thing:2759439

Nor do I guarantee that airflow, static pressure etc. is not affected. I am not an aerodynamics engineer. I just made it a bit longer. It still appears to work totally fine but may not be optimal if you like naval gazing.