Based on jridley's simple and effective design (http://www.thingiverse.com/thing:168586).
This is a plate that attaches to the fan, plus an optional extension to attach it to the printer, with a few customizable options. It can be set for 40mm, 50mm, 60mm, or 80mm fans, and screw hole sizes are customizable. It is possible to choose the position of the mounting tab, the length of the mounting tab, and the angle of the mounting screw. There is also an option for a full plate, or a lighter version that only mounts the fan with two screws.
The default .stl provides for a 40mm fan with a centered mount tab, a 20mm extension from the flat end to the center of the cylinder, and #6 mounting screws.