It's easy enough to make a hollow wax - the trick is to cast it. If
you make a signet ring out of sheet wax, completely enclosing a void
inside, when you cast it you will get a solid ring - there's no way
for the investment to get inside. You can put an investment core in
the center, but then you have to "pin" it in place (there are a
variety of ways) or it will just fall down when the surrounding wax
burns away. Foundries and sculpture-casters deal with these issues
all the time.....