Hi,
If you make a wrapper “a bit larger”, its thickness will be that “bit”. It can be zero. Putting 2 volumes next to each other can make a 0-thick layer in between, that will not be seen in raytracing (this is allowed). Some example of navigating geometry is a recent one posted by a user here:
http://root.cern.ch/phpBB2/viewtopic.php?p=37144#37144
Cheers,