I'm not a caster, but I understand the process, and assume you are
burning out the natural seed pod. If you want a heavier wall on
your seed pods, how about applying a layer of hot wax to the inside
of the pod. Leave the outside alone. If you are casting solid seed
pods, it would not be too hard to make a small hole and squirt the
cavity full of wax and connect it to the sprue.
Just an out-of-the-box thought from Judy in Kansas