I'm not sure about structure, but if you were to dig some small holes in the floor of the pond in about 3 ft of water, and then put gravel in the bottom of the holes it would make a good place for the bluegill to spawn. This would work really well if you had a larger flat where the water didn't drop off that much for about 10-15 feet. If you covered that flat with shovel sized indents and then dumped some gravel in each of them, you'd know where to catch fish during the spawn especially. Now, it's probably to late for this year, but if you did that in the summer, especially if the water were down a little (you'd need to estimate it's normal spring depth so as not be be too deep come spring) I think you'd really have something the following year.
"I come from a state that raises corn, cotton, cockleburs and Democrats. Frothy eloquence neither convinces nor satisfies me. I am from Missouri. You have got to show me."- Willard Duncan Vandiver