This is as close as I can get. I found a picture of someone else's plywood build through google.
The top of a plywood tank typically has a thick brace in the middle to suppot the weight and pressure in the tank.
Rather than just nailing in a tiny piece of plywood, i'm going to use an entire sheet for extra stability. The sheet will have large sections cut out of it and form the same basic thing. For better explanation, I have a lovely piece of MS Paint artwork for you. Assume the two rectangles are even and the middle bar is centered.