This is a remix of the popular semi-circular fan duct for the Anet A8. There are two versions that drop the duct down 3 mm and 5 mm. The 5 mm version works well on my machine without excessively cooling the extruder. Print from ABS or PETG for long term durability. PLA will eventually sag due to proximity to hot end.
1-21-18 Update: Re-uploaded 5mm fan duct with corrections to prevent printing issues.
Printed on a Anet A8 from Inland ABS with 0.1 mm layer height. Had to keep the part fan running at minimum speed to improve print quality for the thin walls.