Radial Fan Duct for Prusa i3 RRW

For a long time now I've wanted to replace the standard fan on my Prusa with a radial fan, which I've read is a must if you want to bring air cooling closer to the action.

This part complements http://www.thingiverse.com/thing:348102 (extruder support with fan holder), is inspired by http://www.thingiverse.com/thing:225126 (cooling duct "that works") and builds on the skills acquired with my teapot lid (design a profile in Inkscape, rotate_extrude it in OpenSCAD). It is incompatible with ht