Special nut with square profile (pair set)

I needed a fully plastic nut with a square profile that will keep the load.
Taking several models and mixing them up a nice couple came out.

My thanks are going to original used models:
- nut with square profile https://grabcad.com/library/pipe-wrench-1
- collet clamping nut https://grabcad.com/library/er11-a-type-collet-clamping-nut-1

The model is designed to fit my designed 2 arms (biggest holes diam 21 mm):
http://www.thingiverse.com/thing:1167051
http://www.thingiverse.com/thing:1