Ryan as far as building rafts to shade the pond it would be easier and cheaper than plywood to use sheets of foam and link them together, they would also reflect the sun well, if you wanted them raised up off the surface just glue some foam blocks to the corners.
Another helpful thing to cool the water would be to run a good size surface pump or fountain which would spray the water high enough to transfer more heat to the night air, an aerator does a good job but doesn't get the water in contact with the cooler air 5 to 10 ft. above the pond.
Good luck and hope your trout make it.
