Flat white paint works well. The white/black plastic as Lothar suggested works great too. I've always used Mylar, even though I've painted the walls underneith it white. *lol*
If you order Mylar, be sure to order the thickest "Mil" you can. Its a little more expensive, but doesn't tear nearly as easily as the thinner stuff. Also, you can staple the Mylar to the walls and it works well.
Vi