This is an alternative design to http://www.thingiverse.com/thing:722274. I do not think it is better than the original (may even be worse).

It has slightly smaller elliptic-shaped entry holes that are easier for my 3D printer resulting in a stronger part (the original used to break rather easy). This is the only reason for redesigning it.

Full FreeCAD source is provided (but it is done with PartDesign NEXT WB, which means you need a very recent FreeCAD, I use it from the freecad-daily Launchpad Ubuntu repository).