Heimo,
You should cut the exact size and form of the wallhole you need. You can check the WallholeCut macro in the library – that simple macro creates all the magic about wallholes for our library.
To be able to create smaller polygons for cutting the wall, you can use the PolyOperations addon, which helps you modifying polygons.
Gergely Fehér
Team Leader, Library Team
GRAPHISOFT SE