So far, I have used two different materials with great success.
1. This is using a Poly-Foam. Not sure how to explain it, but its kind of a Styrofoam but thinner and more dense. It is cheap, easy to get at art supply stores and its very light.
2. This one is using Tamiya's Pla-Plate (basically its a flat sheet of styrene) which I taped onto a used wooden frame. Its light weight but its costly.