This is a Python script for FreeCAD which generates parametric project enclosure boxes. All the dimensions and various other features of the box are completely configurable.
Source also available on gist: https://gist.github.com/1361643
Save the ProjectEnclosure.py file to your FreeCAD macros directory.
From the FreeCAD interface, you can use the "Tools -> Macros..." menu to view or change your macro directory. Once the macro is in place, you can execute the macro from the same dialog.
This script is implemented as a FreeCAD Python Feature which means that you can edit all the parameters from the FreeCAD gui without touching any Python code at all. Select the box, and click on the "Data" tab in the left pane to see and edit any of the parameters. One of the screenshots shows an example of this.