As a possible way to keep osprey from snatching spawning bluegill, I would like to know your thoughts if you built some open bottom boxes with say 12" sides and put gravel in the bottom of them. Would the bluegill go down into those ? Thinking the osprey might not be as likely to bother them because of the high sides causing them to think twice about their normal diving routine ?

In areas most used for spawning you could try staking some nylon bird netting or temporary plastic fencing over the spawning areas. You want them above the water in order to discourage the osprey. They will avoid areas with this barrier. Under the surface, you might trap them which could bring trouble. As for boxes, I am not sure how they would work concerning the BG and/or the osprey. But I think something like netting would be easier to do and provide more benefit for the investment.

I put out 4' fiberglass driveway markers with lightweight mesh on the top 2' for herons. I set them at a 30 to 45 degree angle depending on the water depth, and that's worked well. Cheap and quick to set up and remove.