I have used 3' pieces 3 or 4" pvc laid out near the shoreline.
the male fathead pick a nesting site in many cases 3 or more males will nest on a single piece.
You can place several of these in the pen as spawning structures.
Though I would use the open pond and station the pvc around the shore.
The population of fatheads will increase fast as long as there is food for the young.

What we have done for ourselves alone dies with us; What we have done for others and the world remains and is immortal.