Depending on the size of the shell you may have one or two chambers, on the large shells you will have one in the bottom and one on the top, on the small one, 1/4″ to one inch you likely will have one at the end.
When I use the small shells, I put them in the same bucket with the plaster the sand and the water and stir everything together, so with the mixing motion the shells are partially filled with the Plasterite, but never completely, at the end of each shell there is always a small amount of air trapped, that gives your resonant chamber.
Within the Plasterite we already have a gazillion of resonant chambers in the structure of the Plasterite in it self, what we are doing with the shells is giving the energies a spin in a perfect Phi ratio, making it go further.
When you fill a large shell after everything cures, I have found the energies come out of the small end
Make sense?
Josh