All fan ducts suck. This one sucks less (I hope!). There are several reasons: first, it does not restrict air flow (each branch cross-section is 1/2 of the inlet). Second, it maintains laminar air flow. Third, it provides even cooling from both sides (good for PLA plastic). Fourth: it's completely customizable, just load it into FreeCAD 0.16 and edit variables in cf spreadsheet.
Designed for Anet A8 (Hesine, Omni, etc), but you can change parameters to fit it almost anywhere.
Use heat shield
This fan duct cools heat block a lot, so insulate it or use some kind of heat shield (I made one from cotton stripes and kapton tape). Look at the temperature graph: without heat shield temperature swings cause serious wobble that looks almost exactly like z-woble, but wobbling disappears when heat shield is installed.
There is a gap between heatblock wires and duct's right branch, heat shield fits there just fine.
DO NOT SKIP THIS STEP!!! After installation of fan duct run PID tuning as described here: http://reprap.org/wiki/PID_Tuning with fan running at your usuall fan speed and save new parameters into your EEPROM (or, better, create various PID parameters for various plastics types and set them in gcode startup scripts).
How to customize
Use FreeCAD 0.16 (install from web, don't use stale versions from repos). Open freecad source file, switch to cf spreadsheet and edit variables. If FreeCAD is too slow to recompute on the fly, install Lattice2 extension, turn off recomputes, edit spreadsheet (look at the drawing for variable names), click recompute. Export helix_fan_duct_v7 object as STL.
Note that S3D complains about self-intersecting surfaces. This is due to how FreeCAD handles helix objects, but it does not matter printing-wise. It will print ok.
Print it with "build plate only" supports, upright. Both Cura and S3D support that option. Stock Slic3r fails to slice it, Prusa's Slic3r does work. It's an easy print, done on my junky Anet A8 with stock fan duct. I used PLA plastic, it works OK for PLA printing with bed temperature about 65C and fan 100%. But bed temperature over 80C is too hot for PLA duct, use ABS or something like that.