This is a model based on Motogeepie's amazing design.
(https://www.thingiverse.com/thing:3927320)
After trying half a dozen of various ducts I have settled for this.
I have been using this duct for the past 2 years and I am very satisfied with its performance.
The original design is for the stock 4020 fan but I have modified it to suit a larger 5020 fan for a slightly better cooling performance.
The front half of the duct model (grey in the screen-shots) is intact only the back is edited to suit the larger fan.
Printing:
Front half no supports needed. Rear needs support as well as support blockers inside the air paths. I also have two pictures showing the Gcode preview so you can see where I used supports and where I haven't.
I also used draft shield as I printed this in PETG without an enclosure and certain thin parts would de-laminate without the shield.
I do not recommend printing this in PLA, it will melt because of the bed heat and the close proximity. I know this from experience. ;)
Big thanks to Motogeepie for publishing the original design in STEP format. This greatly helped me to do the modifications for the larger fan. In the same spirit I have also included a STEP file in case someone decides to do further modifications to it.
The author hasn't provided the model origin yet.