The width is okay. But I need a length of 234mm :confused:
It's the same with my outdoor unit; you can either mount two brackets or make new holes in the flush-mounted box accordingly. The bracket does not have to be flush with the unit, it only serves as a kind of "anchor".
Do I have to determine the installation depth beforehand, or can I still change it after the plaster has been applied by adding or removing "rings"? I would first set the base with a few "rings" and cover the whole thing, and only later build everything accordingly.
When plastered, it's finished. Then nothing can be changed. The rings have a build-up height of several millimeters, so you can determine very precisely what your final plaster thickness should be.