10-24-19 Update: Added model with slightly more clearance to nozzle and adjusted mounting holes to go closer in the axis direction toward the nozzle.
Minor update since 10-9-19:
-slightly improved clearance for mounting holes for leeway to mount higher and more snug to fan
-slightly cut short bottom of duct to improve clearance and account for wiggle room from stock Ender 3 fan case that could cause it to be too low
A decent fan duct that uses only the stock Ender 3 parts only. The vents should direct airflow mostly below the nozzle at the extruding plastic.
Make sure not to tighten the bottom two bolts too tightly to avoid breaking the mounting holes (~half a turn once you feel resistance).
Tip: If you find the fan duct rubs against the plastic coming out of the nozzle, you can try loosening the two screws that fix the whole hot end fan case to the gantry, push it up higher, then tighten the screws so the whole setup is slightly higher up.
Liking, sharing your makes, or tips will motivate me to make more things!
Make sure "Support on build plate only" or the equivalent in your slicer is checked to avoid supports on the interior of the duct!
I recommend 3-4 vertical and horizontal walls.
Print quality should turn out well if you have your printer calibrated/tuned to an alright degree.
Heat Protection and/or Roof Surface Fix
Had a problem with my print where the roof surface of the two vent portions of the duct had messed up a bit leaving some holes for air to come out.
I fixed it and added a layer of protection from heat by:
-sanding down the flat surface
-removing any plastic pieces sticking out (from the holes)
-taking a piece of standard printer paper
-gluing it flat on to the surface
-cutting off excess paper
Through trial and error, and many different revisions...
Renditions starting from left to right in a circle