What ever the synthetic glue board that is indestructible and is formed to look like lumber.
Use that.
I've seen that pretty successfully installed and used in the NW, but in other regions not so much.
In the SW, I see decks and benches and the like built of it warped all to hell, they are torn out and replaced with wood rather quickly. Apparently excessive sun or UV does a number on it. Docks built of it can be a disaster, don't overfill that running tank on the dock, a gas spill eats the stuff. Then there is just the whole idea of a plastic deck.....
