Depends...SMB spawning depth depends of many factors, water clarity, temps, etc. To be safe I would place in 3' depth and make the bed 3-4" rock, not sand. Rock allows eggs to remain oxygenated and not buried under silt.
Do you have photos of your structure?
They look pretty much like djstrauder's except rather than a wood box for a concrete form I used some old plastic pots like you get when you buy shrubs, bushes or small trees. I might also make one pallet "tent" and a tire sculpture.
