Would it not be possible to create a single piece object that encloses the seperate piece for the fin, the internal object being seperate but moved by the boundries of the external one, the external one is completely transparent where the inside fin shows through, the internal one shows the view behind it. the single piece object would then not have bad joints and you would control where the internal object was.
Or maybe the external object would be moved by the boundries of the internal object instead.
probably too simplistic a thought
