Ultimaker reinforced low profile external Fan Mount

Dual extrusion support requires the fan be offset outwards a little when used with certain fan ducts.

You can also use this design when the fan must be on the same side as the extruder head.

If you only have a single hotend with the fan on the opposite side then you should be fine with the Mk2 version (http://www.thingiverse.com/thing:31599).