I use 1/2" rigid styrofoam under all my tanks, I don't like to use thicker stuff. Sometimes I have to look around to find it with straight edges instead of beveled edges, but I can usually find it at Home Depot. The stuff I buy is Owen Corning "Foamular". I like the styrofoam to stick out around the tank about 3/16" all around, so since your tank is 24" wide I would cut the styrofoam down the middle of the length and leave a little gap in the middle so it can stick out all around. Using plywood helps to strengthen the stand if there is any doubt, but usually is not needed if the stand is well built and the edges line up nicely. Although if the bottom pane of your tank ever breaks the plywood helps to hold the water in a bit while you're panicking to get everything out.
