1/2" x 1/2" holes is too big to keep them all in the cage except for maybe the largest of the adult FHM's. 1/4" square should be fine for breeding size adults, but would let the teenagers out.
+1 on the hardware cloth being hard on the fish. I did the same thing while culling fish from my pond and within a few days of being in a steel mesh cage...some of their eyes were cloudy from rubbing on the mesh, not to mention the scale scuffs and scratches.
