Would like to create some areas for bluegill to spawn - so

1) depth spawning areas should be placed ?
2) use gravel ? if so, size preferred ?
3) Should it be placed on the pond bottom or should I build some sort of large wooden boxes and fill them with rocks ?
4) How do you minimize predation by Osprey ? Bluegill now spawn in about 18" of water in my pond and the Osprey are continually picking them off their beds.

Looking forward to input