Have you thought about painting it? You can get a grain effect by dry-brushing with both a darker brown than basic color and with a touch of black now and then. If your public library has a book on theater technology many of them show such painting techniques, like "Scene Design and Stage Lighting" by Oren Parker and Harvey Smith, later by Parker and Craig Wolf. They have some VERY cool painting techniques that create the illusions.

Or you could paint it off-white and add 2x4's at 45 degrees to look like British Tutor wood and plaster.