This is a duct to use a 5v 5015s centrifugal fan (from ebay or amazon) blowing through the hole above the extruder stepper motor. The duct just clips into place and need no extra attachments, the air passes over the extruder, past the hot end heat sink and through the existing curved duct down to the hot end, creating a layer fan (of sorts!) I got the fan from ebay, its a 5v 5015s centrifugal fan and costs 99p (in the UK!) The photos show an early version printed without the fan, which I used to print the second version... with much improvement! I use this in conjunction with the extruder fan shroud http://www.thingiverse.com/thing:856155 which means that all the air goes down the duct behind the hot end. You get quite a breeze under there now! Wire the fan in parallel to the extruder fan (5v) so it is on when that fan is on. If you install repetier then you can repurpose this to be a layer fan. Just don't turn them all off or your filament will melt in the feed and block everything! There are circumstances where you don't need a layer fan, either unplug it or turn it down in repetier, just don't turn them all off. So far I just leave them all on and up the extruder temperature by 5 deg. I have found that the existing extruder fan runs a lot quieter, this will be because it's not pushing air through as efficiently, no doubt I will need to replace it with a centrifugal fan as well, once I work out how to get it to fit and not foul on everything when in the home position! Print it upright with plenty of support. You need a layer fan to get the top to print without drooping! I print at 6000mm/min with 20% infill and 35% speed reduction for layers < 5 seconds (so the top prints at around 600mm/min or 10mm/sec)