This fan duct design widen the bottleneck of the old one:
http://www.thingiverse.com/thing:980654
Its disadvantage is that airway turns stiffly. So you better use strong fan with high air pressure.
Cr7FanDuctRingV4_5.0.5.3.stl | 111.7KB | |
Cr7FanDuctRingV4_conjV2.stl | 17.2KB |