I designed this duct because the "stock" ducts from Guanu, John, and HeathenX wouldn't fit under my platform (custom E3D mount). It was also an exercise in re-learning to draw in CAD after many years of rust. This fan fits the "standard" 10x30 blower fan that SeeMe sells, and fits in the <8.5mm clearance that I have between my effector platform and the tip of the nozzle without risking snagging on a warped part. I also tried to use natural curves to promote laminar flow without turbulence (eyeball method only) and flow as much air as possible without putting unnecessary back pressure on the fan itself.
V2 (12/7/14) Includes better bracing on the mount tab, smoother internal curves, a different opening on the bottom of the duct for more downward airflow, and side "wings" to encourage more directed airflow. The model also is optimized to have a 1.5mm wall thickness on all of the angled pieces in relation to printed layer angle (more extrusion width friendly). It's also drawn up in DesignSpark and is a much happier model that the previous version that was from FreeCAD.
This mount has 1.5-2mm thick walls, so it's not terribly wasteful to print it at 100% infill. You can experiment for yourself, but that's what I did. Good Luck!
Added the air deflector ring. It's not really amazing (wouldn't call it a finished design), but you might find it useful.