With the sides around the bottom the sides act as the rim of the tank. You do not want any foam under a tank build with that method. I would not do it myself, but I try to take the safest route possible.
Does this stand look sturdy enough to support a 24 gallon tank? It's advertised as being made of rubberwood and poplar solids. I'm mainly worried about the middle sagging because it's only supported by the 3/4 inch thick wood top. How strong is an 18 x 18 section of solid poplar / rubberwood?