UPDATE 21-05-2015: Updated photo renders, STL fixed, updated source file.
Because I did not want to place two fans around my hotend, one for the extruder cooling and one for the filament cooling, I decided to design a double fan duct which can cool both with only one 40mm fan. Results are pretty good, all prints come out great and I have not had any filament jamming so far.
I have uploaded, next to the source files (Solidworks), two STL versions. One is designed to fit on the official E3D V6 hotend, one is designed for my Chinese E3D clone. They are both the same, except for the with and height of the fan mount. The official E3D has a diameter of 22.3mm and a height of 26mm, my clone has a diameter of 25mm and a height of 32mm. To check which version you need, just measure the diameter of your hotend. The length is less important as the diameter.
Mounting is easy, just screw a 40mm fan with 4 M3 screws to the fan duct, click the fan duct to your hotend and you are good to go! The click-on system is really simple, but still very strong. Even with strong vibrations the fan duct on my printer does not move on the hotend.
Printing can best be done by rotating the print 45 degrees and printing the fan duct with the fan mount to the bottom.
The designs are with a GPL license. So do what you want with the files, as long as you publish your modificated object under a GPL license and give the correct references.