I'm in the process of doing the same thing...
DIY - Brake Cooling Duct Installation
My DIY is more of just plain pictures but I'll update you guys when I'm done. I decided to go with 3" Diameter...a little overkill, in fact now i'm a little worried because more cool air will get exposed to the surface than the inner hub of the rotor. Oh well...maybe I'll design something else like a tray funnel style one day.
Also, have you taken any temperature? Stillen's website (essentially the exact same thing) claims 25% drop in temp, but I noticed they put a Stoptech brake setup. Who knows if that claim is really true.