try balancing a mushroom with no soil. a thin base needs wieght depending on the size of the plant
I think that would be a good mix for a bubbler system.
I would only use the lava rocks if you need the wieght there are other ways to secure a net cup for a large plant. I dig many small plants myself. with aero you can get a ounce a plant in a 9inch square area with a 12 inch plant.

One love
c