In spite of finding plenty of designs for fans, none seemed to work for me. Some would cool the hot end so much it got it to a temperature so low it would stop extruding, whereas others wouldn't cool it enough. This led me to design my own centrifugal fan based off of a small DC motor commonly found in cheap toys. If you are picking a motor especially for this, make sure it's got high rpm (torque is of little importance here). Furthermore most of these motors will not be 12V tolerant and so will need to be either hooked up to an appropriate voltage source or PWM limited in software to match their rated voltage (minus a few percent, just in case). I used the parametric centrifugal fan rotor design found here: https://thingiverse-production.s3.amazonaws.com/assets/03/81/3b/4a/f8/centrifugal_fan.scad. As well as the model for a hexagon found here: http://svn.clifford.at/openscad/trunk/libraries/shapes.scad. The fan runs well for me as the exhausts are perpendicular to the print bed and it doesn't cool my hot-end like mad. Some of you (especially those with cartridge heaters) may wish to modify the angle of exhaust holes in the hex tube for an under-the-nozzle air exhaustion experience. If you print the STLs as they are, the tube will need filing else they might delaminate (which is what they did for me, shown in the pictures) as you mount them on the body of the fan. I added no tolerance here on purpose so a tight fit can be obtained through trial and error post-printing. The SCAD file is rather messy but quite modular. Please note that new STLs generated from the .scad file may require repairing in Netfabb before they are printable.