It was the first try, and I think it worked out quite well (sorry, this image is a bit blurry). With the remaining PC I also built the mount for the backpack.
While the PC cured, I looked into the issue with the shrine door and the FDO again.
As you can see, the opened door blocks exactly his position.
So I removed the right wing of the door (left on the image) from its hinges.
and I will add a central hinge, so the door becomes a folding-door.
Luckily I kept all the pre-cut pieces.
So I could rather quickly assemble the central hinge.
This is the inside.
The outside, closed.
The door opened and the right wing partially folded.
And the whole door in the final open position.