This is the result of an experiment in making a cheap fixture for 4-sided (or more) milling fixture.  This was originally introduced here and is published in case anyone wants to pick it up to experiment further.

It can be seen in action here: 

You will need OpenSCAD to modify the files and the design is pretty well parametric, although it is not necessarily cleaned up for easy use.

This comes with no warranty whatsoever.  Please be careful.