June 6, 2017: Updated version 1.2: Simplified and lighter design. Now just 6gr.
June 5, 2017: Updated version 1.1: Slightly simplified and smoothed revision.
Following a short research, I found the the design by Lokster is considered the most effective, so I downloaded and started improving based on Lokster's basic geometry.
The improvements in this design are:
a. Bigger airflow - by increasing the duct cross-section & enlarging the air holes.
b. Better air distribution - by installing internal air guides to distribute air equally to all holes.
c. Better air direction - by adding air nozzles to point blow the nozzle and avoid air flow to the hotend.
d. Easy printing - no supports required.
Being printing PLA with it ever since, with great results.
Great for printing at high speeds and/or layer heights and/or temperatures, as it provides the airflow to cool the big flow of PLA.
Please print the duct slow (to achieve smooth internal surfaces) and DO NOT use rafts or supports!
Printed mine on translucent PLA - very bad for showing the design, but helps a lot with the led strip I installed just above it...
DO NOT use rafts or supports (maybe just on the outside back side), they are not needed and will be impossible to remove (from inside or through holes) without messing the design.