Tracy, edges should probably be your biggest concern with blocking nets. The bottom, banks, and surface can all be escape routes if proper setup care isn't taken before you add any fish. I've yet to set up a blocking net that didn't eventually open up from limbs on the bottom of the pond, quick water level rises that lifted the net off the bottom, or windy day tree trash that fell on the top line, and dropped the net below water level. The majority of fish did stay in the blocked area, but there were always interlopers of some kind that showed up for a meal, and targeted fish getting out.
TJ and Bill obviously gave you great info. The cages and blocking net nets will algae up, and I've had good luck with a few single sexed tilapia thrown into the mix.
I'll try blocking nets again, and it's a fun project, but IMO neither a blocking net or cage will produce the pure numbers a dedicated water filled hole in the ground will. Ultimately, I would think you're wanting thousands, to tens of thousands of 3-4" CNBG, and I think that's best accomplished when done in a single species dedicated pond. In essence, this is a hatchery project, so that's probably the fastest and in long term effort the cheapest, so that's how I would go.
