I tried to create the real curved cutout. for this you need an invisible box which is bended the same way your wall is.
This is not that easy as i thought.
I had to take the piece of wall i want to use for the cutout and export it as new furniture (as usual), but furthermore i had to remove the front part of the resulting object. Not sure if everybody here can do that as well ... [:(]
anyway, here's the picture:
feel free to give ideas to solve this in an easy way (we need to bend an "invisible box")
macfrog