This is not meant for 3D printing, although I guess you could. I'm learning FreeCAD (0.17) as a hobby and this was an exercise in using the Part and Part Design workbenches with their various tools, and a little spreadsheet to contain the parameters. By using an internal spreadsheet I can quickly adjust the radius, height etc. of the fan.
If there's any interest at all I can describe the workflow more precisely.
FreeCAD file is included. Comments are welcome.
At least one person already made good use of this design: https://www.thingiverse.com/thing:3017255.