Here is a thread with some pictures of the type structure I build for FHM. Lots of "underside" area with different places so the FHM can have their "territory". I try to space the layers of boards 1 to 2" apart. This gives some protection from larger fish predators while providing lots of spawning "areas" in a compact structure.
Just built two more of these for the sediment pond I built recently. Will let the FHM's populate it before putting in any other forage fish.
Pallets will float in the beginning then sink. So on the later versions I added a couple strips of foam insulation on top for flotation when the wood saturates and some rock on top of that to get the unit to sink further down in the water while the wood is still buoyant. Then I tie them off with a rope attached to a stake along the shore. This allows them to rise and fall with water levels yet stay in one general location. I can also pull the unit to the shore and pull it out of the water at the end of the season.
They appear to work as my mini-forage pond now has gazillions of FHM fry and my main pond (which I also built lots of structure for FHM) has masses of them that I can see scatter at the top of the water as I ride the 4 wheeler around the pond.
I've also noticed the FHM will excavate under some of the 3" rock I have in the pond and forage pond and use the underside of this rock structure. So they will use about anything solid that has an underside.
Old thread about FHM condo 
