Credit to SpannerHands for inspiration, and for his nozzle, which I have included here unaltered for convenience. (If you are looking at nozzle alternatives, you should read Desi Quintans nozzle study: http://www.desiquintans.com/coolingtests.)
However, I didn't care for the sideways mounting of the fan, so this duct moves it to a vertical and fore-and-aft position.
(Incidentally, I sliced this using pretty aggressive settings in Cura's Fuzzy Surface feature; that's why it looks... fuzzy... in the picture.)
ETA: In case your slicer doesn't have a "lay flat" tool, I've added a copy of the STL rotated into proper printing orientation.
ETA: I was experimenting with Sigsmond0's cable chain (thing:1060522), and the part E_motor_mount just barely clears the duct. I added a "more clearance" version to move the fan out a few millimeters. I have left the original, but recommend using this version, as there doesn't seem to be a downside.