I took MiseryBot's fixed 60mmx25mm fan model from http://www.thingiverse.com/thing:8063 and made it parametric so I could use it for multiple fan sizes.

Not for building, just for modeling. The STL file is just for Thingiview to work, you want the SCAD file!

I use it in combination with the % operator to help me visualize the end result but not be included in final renderings used for printing.

Include the module with a "use" function at the top of your SCAD file. Create the fan by using the fan module and passing in the width, depth, and mount spacing as parameters in milimeters. I plan to make a set of modules for commonly sized fans to make it easier to use, mostly for the mount spacing because that parameter isn't obvious like the other two are. For now you have to provide the values. There are some other optional parameters to control how rounded the corners are, etc. Look at the source to see them all.