Summary "Is there a way to get the interior empty space volume of a top assembly?"
Pro Gurus: Thanks for all the inputs and I apologize for my very late summary. I was burried under two hot projects.
Anyway, it seems that shrinkwrap is the proposed method for doing this. Based on its short description, solution #4 seems to be the simplest and quickest. I wonder if I understand the procedure correctly. First create two shrinkwrap model. One will be filled and the other one won't be. Finally substract one from the other. I am still confuse of how to do this and will play with it when I have a break.