Around here we can get either 3/8ths or 5/8ths treated plain plywood that will last a LONG time. I have a couple of scrap pieces laying out in the weather that have been there for years that you can't tell any change in. We can also get pre-painted camo sheets and the price isn't too bad. With any salt based treated wood you need to use either stainless screws or hot dipped galvanized.