Dual Extruder fan duct for Geeetech I3 Pro C Dual extruder

[update: 06.06.2017]
i recently uploaded a new version using two radial blowers and one axial fan:
https://www.thingiverse.com/thing:2369048

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this is a remix to http://www.thingiverse.com/thing:1337672

* i added pillars between the surface and the shrouds, so it is more stable (see comment from "made by Pyroplan")
* i also added a hole to fix it to the printer with some cable ties
* if you want to use a third fan in the middle, i made a 40mm gap be