SMB like to utilize spawning structure that has 2"-3" rocks in it with a backstop on the shore side of large rocks or a large log. They will spawn on pea gravel, but the coarser material is better.

I have had great success in using the pallet, #2 limestone and cinder blocks like TJ posted years ago. I have had SMB start using them within a few weeks of putting in the pond when water temp was 60°F in the Spring. 2'-4' water depth. I used plastic netting on top of the pallet to keep the rocks from falling through the openings and plastic pallets are best because they don't degrade over time.