Honestly, looking at what you have, if you set it on plywood and then styrofoam your going to see that. You dont have a lip of any sort to hide that stuff. Plus I thought the point of the plywood is to support the weight when there isn't a rim around the bottom.
I have plywood under mine but it was hidden on my stand by a 1-1/2 inch lip and it was just to raise the tank up in the stand. I also have styrofoam under my sump because it is a acrylic box with no sort of bottom rim so the foam evens out the weight or levels it or whatever.