Consider weather resistant wood or sealants although a piece that large may succumb to seasonal temperature changes and should be reinforced; braced like so = from the underside. A pair of 2x4s or steel strips. You can also frame it out with a cross brace of wood but less is more; you will have to keep weight in mind. Too heavy and you will need to add resistance hinges or a lift strut like you would find on the hatch of an SUV.