CDX sheathing (half inch) is the cheapest way to go. If you have more money, go with 3/8 or 1/4 ABX, which will be lighter, and will behave better if it gets wet. If you have letters that don't "stand up" by themselves (like Gamma), put a piece of aluminum tubing on the back. Then, poke a piece of steel conduit in the ground and slide the tubing over the conduit.
And, if you really have the money, marine plywood is the way to go.
This is an election year, which means
http://en.wikipedia.org/wiki/Corrugated_plastic is everywhere. Just grab a few signs (hopefully after the election). Laminate them with RTV, and tape the edges with black duct tape.
Hybrid Mode
