What if you approach this from a different angle.

What if you make your cage and only put in a handful of FHM's? Maybe a couple dozen. However many could have spawning areas they could protect. Size the mesh of the cage so the breeders stay in (and anything that could eat them out) and the recruitment (babies) wanders out into your pond.

Provide cover and habitat for the fry around the cage.

Could it be you would get more breeder production that way?