Hello mova

I have the same "problem". The whole thing has a lot of play. I also fixed the number with cable ties.
Since the number plate holder swivels upwards when folded, you should make sure that it still works when you screw something on.

Hello mova
I have mounted a very cheap exchangeable number plate holder (long). Only when folding it up do you have to push/help the outer edge of the frame in a little at the very beginning so that it doesn't hit the holder. However, the construction is stable and nothing rattles.