Hi Sven!
I haven't seen your scene or know anything about it, but it could be
that two of Mike's excellent tutorials could help you with your bounding
issues:
Mike's isosurface tutorial:
http://www.econym.demon.co.uk/isotut/
Mike's holes tutorial:
http://www.econym.demon.co.uk/holetut/index.htm
HTH,
Florian