This is how it turned out. Quite amazing, right? (I now have some kind of experimental config that started out with two bass strings and two guitar strings, that I put on when I last actually used it for a no-bass trio in 1995.)
This is what we had to start with, so we layed it on the plywood and drawed around it.

But I did wanted it to look like this:
Lesson learned: never ever trust a grown-up.