t-nut / slot nut for openbuild v-slot extrusion

I tried to improve the design:

- Version 2: rotatet the hexagon for the nut around 30 degrees and made it a bit smaller, so you don't need to glue it

- Version 3: rounded the edges to make it tight possible to slide them in, when the plastic corner is already in place.

- Version4: made a "hole" which is 10.1mm in diameter and 0.9mm deep to insert a washer.

It doesn't work perfect but it works. Maybe someday i will work again on the shape to make it easier possible to insert them.