Here is a remix of Lion4H's Radial Fan Fang to be able to mount to a 40mm x 40mm hot end fan instead of an E3D 30mm fan. Specifically designed to work on a Creality CR-10 with superpotatofudge's hot end cooling duct and the stock hot end and cooling fan. Requires the use of a 5015 radial blower fan like the original design. Blender file included to allow modifications, just look at Lion4H's thing for instructions. Probably works for afinitbot and CR-7/8 as well.
Updates:
4/9/2017: Thickened fan mounting bracket that connects to the hot end fan as original was a little thin and was buckling when tightened down along the bottom of the mount.  Haven't tested yet, but shouldn't be any issues with this version.
4/29/2017: Version 2 of the no probe fang duct that mounts between the fan and superpotatofudge's hot end cooling duct. A little thicker for durability.
5/7/2017: Added new version that thickens the 5015 mounting fins.
5/27/2017: Updated photos to reflect how the thicker version should be installed. Pictures…

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.…