You are correct, the stand grows a little when it is sheeted with light ply. I made the frame the exact tank size, intending the top 3/4" ply sheet to be 1/4" bigger in all directions. The 1/4" side sheeting will slip up under the top piece, and the top piece will be flush-trimmed with a router. Then I will mount molding around the joint, and up the side of the tank some to cover the sand bed.
Attaching the wood to the metal is a little tricky. I did not want to drill and screw, as I had just paid good money to powdercoat it. What I did is sandwich it on all sides. I have only done this on the bottom, as shown in the pictures, so far, but intend to sheath the sides and top the same way.
I will cut 1/4" oak ply to fit on all sides, and glue/brad all the corners together as needed. I may also use liquid nails to hold the wood to the metal.
Sorry I can't get you some pictures yet. Ask away if I'm not being clear yet.
Zeph