I've had great success with buckshot or pea gravel (1/4-3/8") obtainable from any garden supply center. The bluegill male will fan out a depression doing a tail stand. The gravel affords some measure of protection for the eggs as they fall between the gaps. I would put a rubbermaid-like container with some size to it in there (not a deep one) with about 4" of pea gravel. After the male coaxes a female into the nest area and spawns with her (and other females present), he will hover over it, guarding the spawn. I've used this successfully with several different species of sunfish. I think the trigger water temperature is around 74 F, at least for Longears.