I use a wool pad on top of my ironing board. You wouldn't need a cover over that but I think you would need to fasten the wool pad to the plywood in some fashion. If you spray with water, there is always the possibility of getting the plywood damp. Plywood is made by gluing layers of wood together so getting it damp is not going to be a good idea. You're going to risk warping. It might be better to use a solid wood plank instead of ply wood.