I wanted to use the jigs mentioned here: http://www.gundersen.net/charliecube/
but only GCODE files were available. And there were sufficient differences between the Felix printer they were created for and my RigidBot as to make them unusable. So I designed my own using OpenSCAD. Note, however, I am still (6 weeks) awaiting my PCBs so the final dimensions will need tweaking. But it's a lot easier to tweak a .scad file than a .gcode file. Updated 2 Nov 2014: final files.