This fan duct is valid for every 30x30x10 fan, but the lenghts are designed for the Kossel clear fan mount: http://www.thingiverse.com/thing:372568 This duct is for cooling the extruder. Whithout it, the aire blown by the fan goes to the nozzle, cooling it too, in a way that it's impossible to reach more than 250ºC and slowing the heating. The purpouse of the fan is avoiding the heat going up through the extruder body and preventing the jamming of the plastic or damaging the extruder itself, I think. Enjoy! Maybe if you have wires from the resistance (heater) and thermistor, you will have to cut a chamfer to the closest part of the nozzle. I made it with 0,2mm layer, 35%infill, with 0,5mm nozzle and 4700mm/min printing speed and 75% speed outlines. I always use 2mm brim to avoid wrapping. Fix it with M3x16 screws, it have two holes to put the hex, but I fount that they are unnecessary.