That is how I did my panels: styrofoam backing in between the bracing.
The advantage of this system is that you can use my non-patented stick-pin method of holding terrain down on it. Flock some tacks and stick-pins as trees and bushes and use them to pin down mobile terrain pieces.
The holes the pins leave are hardly noticeable and you can always paint of flock over them ever three or four games or so if they really irritate you.
We get slapped around, but we have a good time!