20x20 Sigma Nut Holder

There are nut holders for M3, M4 and M5.
I prefer M5 nuts for 20x20 sigma profile because I fit my nema 23 motors by them.
If you just attach small parts, M3 can do the job.

You can use these holders with this corner bracket: http://www.thingiverse.com/thing:1934326