Anyone know where to find a *cheap* waterproof cover for a body pillow?

DS is mostly potty trained and only has accidents at nap time occasionally. He's on a floor bed so we gave him an old body pillow to keep him from hitting his head on the wall/baseboard. So far he never managed to get it wet, but today he did. I don't want something that cost more than the pillow itself, but I'm not sure he's ready for something smaller, and it is hard to wash.