You may wish to consider a very water resistant OSB. I know, i know... people try to avoid "particle board", this is not. OSB is extremely strong versus plywood and now a days it is the industry standard in the building market. Marine ply still soaks up plenty of water. Marine ply is a decay resistant product. It's all about the resins and chemicals used during manufacture.
I do recommend this product. You can take a piece and drop it in a bucket of water and actually see the water resistance. Pull it out after a week; visually it shows no absorption. Avoid any type of regular OSB. MDO i would avoid as well as it can absorb water as well if any gets to the cut edges or penetrates any surface openings.
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You can find this product at most REAL builder supply places, or order a sheet in. ie. not Home Depot, Lowes. Try a roofer supplier. It is expensive though.
It may not be the best for visual though. It does not bend at all if you need to shape it.
Using "exterior" grade plywood would not last long at all! It's a common misinterpretation that it will hold up to weather/water.
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