lol. 600's are fun and you have lots of stocking options! Save some cash and don't worry about using exterior grade plywood. I used 5'8" ply on my tank still no problems over a year later. Make sure you use deck screws and not drywall screws. Drywall screws aren't meant to hold constant weight like that and the heads may pop off. Drywall screws are ok though when you screw the plywood to the 2x4 supports around the tank just not when putting the frame together. If you look at my sig I have th plans made up for you already and a step by step thats what le patron meant.
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You can cut costs even further and forgo the fiberglass mat. I used regular automotive car bondo to fill in all the seams and cover all the screw heads it bonds very well to wood. good luck!