Check out Homasote, information available here: [link=http://www.homasote.com/]Building Board Material[/link] If you really want to get fancy, purchase a piece of 1/4 inch thick glass, 18"X60" (a good size - it can be ordered thru any competent glass shop) and glue the Homasote to the glass. Why glass? Because it is flat, flat, flat. But you get a bonus with the glass. Just turn it over after you are done building and now you have an excellent surface for cutting cover material on.