Hi all, I’m getting the same problem as Jono with the Nyloc nuts spinning in place on the metric version. I’ve solved it with this: http://www.thingiverse.com/thing:1904014 “M4 Nut Encasement”.
Basically, it’s a little casing that holds an M4 nut and makes it fit closely into the hex cavities, where the nut would otherwise spin. They need printing with a fine nozzle because the walls are so thin. I’ve posted the OpenSCAD file – it’s very simple; they’d probably work better with M3.5, so feel free to experiment.
Hope this helps!