I'm currently building a stand for a 75 gallon and went with RocketEngineer's design, on
reefcentral.
I attached a photo of mine so far. It's sturdy as **** and I'd say doesn't need the plywood to stay square. It has tons of strength where it's needed, is efficient with materials, and the 2x6 runs give more room for the opening because a center support isn't needed.
I'm going to put 1/2" plywood on the top and I have 1/4" birch plywood for the sides. I was tempted to do teak to match our mid century furniture but I couldn't find Danish teak.