Bluegill love to spawn in pea gravel. Make beds in water no deeper than three feet, preferably less than two feet. I think it wise to put plastic or shadecloth or something similar on the pond bottom, the put pea gravel on top of the plastic, etc. Gravel will stay put for longer. Remember, bluegill are colony spawners, so put a large area for 20-30 crater-type nests to be made.
I'm assuming your bluegill aren't using the rocky areas you already have.

Teach a man to grow fish...
He can teach to catch fish...