FHM are prolific spawners and will use the underside of literally anything to attach eggs. Vegetation and any structure [trees, stumps, pvc, rock, ect.] present in the pond allows them plenty of opportunities to spawn. I also placed shallow tires [1-3'] in my trophy panfish fishery and the FHM are ubiquitous. You can add structure specifically targeted towards FHM spawning, but my experience dictates it's not necessary provided the BOW is devoid of predatory species - they will do just fine if you allow them a few months to reproduce and have some vegetation, rock, tires...but if you're looking for projects, adding specific structure certainly won't hurt.
Snrub is correct - think shallow 1-3' depths, they are territorial, and feeding helps increase reproduction. Well done, rub!
Many men go fishing all of their lives without knowing that it is not fish they are after. ~ Henry David Thoreau