For those of you asking for an Ender 3 "PRO" Bullseye duct to fit the wider attaching points for the 4010 fan on the PRO, here you go. It should accommodate the extra width nicely. This also fits the CR10 Mini and the 2020 version of the Ender 3.
The file is:
Ender3ProBullseyeDuct10.19
Dave
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2/25/20
A user messaged me that this will also fit the Cr10 Mini which uses the same fan as the Ender 3 Pro.
Print with 4+ walls with 50%+ infill. These are structural parts and need to be strong, especially the mounting points.  

9.27.2018  I adjusted the duct to have a little thicker arms around the holes and changed the larger inlet so you won't need a calibrated printer.  
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I’ve had some questions about the duct height and the Petsfang v2 and Bullseye. I decided to borrow a colleagues' smoke generator to give a better visual of the airflow since the "water tests" only show the turbulence.…