So, I just finished up running the next simulation with the 650 gallon tank and 15/32 plywood and things look pretty good. A max stress of just under 3000 psi puts us in pretty good shape. I didn't bother correcting the load distribution on the sidewalls. If its good now, it'll only be better with less loading on it.
Here is the stress distribution. I'll probably add some corner supports like Gary did with his, especially since I'll be using thinner plywood. Without something solid to screw into, it'll be hard to make a strong joint.