... I can't seem to conceptualize exactly how it would work to cast (or have someone cast for me) a hollow piece.
You might want to check out the following:
I had to read it a few times over to let it sink in because the
process does not seem intuitive for some reason.
In the end though the fog clears and one can see what is happening:
investment replaces the WSW and that’s what allows the hollow shape
to be formed when cast.
Cheers,
Trevor F.