What corresponds to the walls in the expanding universe, Leigh?
There are no walls. In the case of the closed universe the normal
modes may be looked upon as closed loops, in which case the number
of nodes in each is preserved as the loop expands with the universe.
Beyond such handwaving arguments I'm afraid I can't go. Of course
there are no walls in the open universe.
All of this is very nearly in the realm of metaphysics rather than
physics in my opinion.