Finally it worked.
Thanks for the advice, not to mirror the stl model. But still your first picture in the first post is my favorite.
I wonder how it looks in side view - like a zig zag?
The radius of my endmill was enough to keep the triangle in the material - I didn´t use hot melt glue. But I had tabs in the corners.
It takes some time to clean the inside corners. Im done with only one of them