EDIT: New ducts.
TAHfan15 is a front duct for 40 mm. Good performance with a strong fan.
TAHfan16 is a rear duct for 40 mm. It shouldl be attached to the backsideof the bottom bearing block with a single m4x10 screw. Can be used together with the front duct for double coverage. I like to use this one on its own now, without a front fan, to better see what the printer is doing.

TAHfan17 is a front duct for 30mm. I have not tested this yet, but I expect it to be far superior to the stock duct due to the smooth channeling and larger opening.

OLD:
Straight duct that envelopes the hot end more than 180°. I might add a second duct on the back of the extruder. It's being devloped and tested. So far it works pretty good, but most of the flow is coming out of the sides.