All good info here, but I'll add what we do in Kansas Hatcheries for FHM production-easily duplicated in a simpler manner..
We drive fence post staples, 2 at each end of a 4'X8' sheet of plywood and run braided cable through the staples and float 20-30 sheets on the surface with cables staked on either side of rearing ponds.
I used wood pallets on my pond and used eyelet screws to run 1/4" rope through. If you think they will sink (I've never had one sink anywhere I've done this) run a pool noodle through each side.

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