I use white .06 sheet styrene (available from Evergreen, Plastruct, or most hobby centers). It comes in a variety of sizes and is easy to mark, cut, or scribe & break and does not deform with normal use (stay away from the finished pattern with X-Actos, tho!!). For large patterns I use a chipboard sheet available from Hobby Lobby. I also use the foamboard occasionally like bikerd69.