I decided to go with 1/4" plywood. It's cheap and light, however it requires a little more bracing. I cut three sheets down to 3' x 7' and glued/screwed the 1x2 bracing in place.
I drilled a series of holes along one side of two panels and both sides of one panel in order to bolt them together. I numbered each panel so when I lay them out I know which side butts up against the next.
I reinforce the entire surface when bolted together by bolting a pair of 2x4s across the panels. I numbered these attachment points.
Once bolted together and reinforced, I simply lay the panels on a pair of 6-foot folding tables and clamp down to keep them from sliding (don't blow your back out trying to do this step on your own; get some help).
I chose for my first game to lay my mat (spray painted fleece) over my hills, but imagine I will place hills on top of the mat sometimes.
For me this is a pretty good alternative to having a table set up all the time (which is not an option right now with kids that like to run around the basement). I also like the ability to use two panels if I want a smaller surface or eventually add more panels for a longer surface.