I have experienced with extra fan but find them produce too much noise. I used NAPALM's designs as the base and fit around the heat sink with an added cable tight on the top. The Duct can also be double sided tape down to the top of the heatsink. I dont seem to find it work any less than external fan when machine running at standard speed. I dont run my Geetech Prusa i3 over 50 mm/s as the vibration is too high anyway with acrylic frame even with the Anti-vibration feet installed. It a lot quieter than having an extra fan.

Update 2017/01/31
As requested a reduced 10mm version uploaded.

Update 2017/03/09
Matching edited 3d Touch Holder, Distance from Geetech design, offset X= +15mm Y= -3mm.