Hey Mumma Bear, you are on to something here. I tried it, and you are right, more swelling and rounder feel/appearance.
I used a plastic bowl that had a flat bottom, I cut off the bottom and it fit nicely into my XL dome, eliminating the last 2-2.5 inches of dome space. I used duct tape in an X pattern with over hang to secure into the domes, making a small hole in the center to allow air through, as the bowl fit so nicely it probably would not allow good air flow.
Like you said, it's so simple and now I understand why philli used the smaller chineese domes. She mentioned it was purely for "shape", and I remember her pictures showing her breast flattened against the end of the dome. When you stop the breast from moving outward, they will push out and swell to the sides, bottom and top.
I only tried it in one dome, but it definately works. I'm too busy at the current time to deal with it, but it is something I will try later. It really does work if anyone is wondering.
Basically, depending on the size dome you are using, having domes the same size circumference, but not soo deep, would do the same thing, basically allowing the breast to "expand" rather than "extend".
Good Job and thank you the tip!