EDIT - 20.12.2016
This cooler is still popular. I have changed the licence to let others adapt it to new printer designs.
EDIT2 - 29.11.2015
Updated again. More volume in the ducts and wider spacing between the exhaust openings. I hope it still performs like the old one.
EDIT - 29.11.2015
New design with a new blower orientation to clear obstacles better. Also has supports for the openings. I am printing it now myself. It doesn't need as much support to print. The walls are slightly thicker than before, but not quite as thick as the previous "fat" version.
EDIT - 31.10.2015
Added a duct with thicker walls. Only choose this if you have problems with printing the other one.
EDIT - 27.10.2015:
I added a picture of my own 70-80° overhang test which tests around all four sides, and it does equally well on all four sides. Also uploaded a duct with a slightly offset mounting hole to better align with the nozzle.
EDIT - 26.10.2015:
New and improved version with smoother ducts and more airflow directed towards the rear. The airflow feels very centered around the nozzle now. It performs better than any other I have tried on the Atom 80° Overhang test now.
A powerful cooler for 50x15mm blower fans. Being rear mounted means there's nothing obstructing the view of the print. The ducts are shaped to deliver a powerful blast from two sides without cooling the hot end too much, and a strong air flow can be felt all around the nozzle.
It mounts in the lower left hole of the lower bearing block. I recommend printing my plastic bearing blocks ( http://www.thingiverse.com/thing:1172838 ) and replacing the aluminum ones with them, because it lightens the printer considerably.
If you want to use the stock bearings, the hole might need to be tapped with a regular M4 tap, though you might get away with just threading in the screw if you use a shorter 10 mm screw. There is no need to remove the block to tap the hole, as there should be no screw in the hole originally. If you feel the flow is not centered around the nozzle try printing the little spacer to move the duct back a little.
Mounting it is not rocket science, and if you need to make some mods to adapt it to your printer because your printer isn't 100% equal to mine then deal with it. Just do it.
12V 50x50x15 mm blower fan. I use a cheap bushing fan with 0.18A rated current draw.
Here's a very cheap one that should work fine:
1x M4x10 mm to 12 mm screw for mounting the duct to the printer.
1x M4x20 mm screw for mounting the blower to the duct.
Here's a handy assortment for this and more projects:
Maximum 0.4 mm nozzle size.
Use supports to the bed as necessary depending on your current overhang performance. Don't use internal supports, obviously.
I printed the prototypes using PLA, and they didn't melt despite printing at 230°C, but ABS would be safer.