I wanted black aluminum extrusions, but in my country (Germany) black extrusions are only available with a 5 mm groove (i've made a model of my extrusions: https://www.thingiverse.com/thing:2273455), so the M5 hammer nuts can't be used, because they need a 6mm groove.
This is my low cost workaround: print this helper and press M5 hex nuts with half height (DIN 439) in it.
I've attached the Freecad file so you can alter dimensions if you want.
The helpers worked for me, i used hundreds of them for my Hypercube build. I've printed them in ABS.