Have a look into phenolic plywood. AFAIK it is much more durable than even marine plywood, it used for shuttering concrete which can be re-used and it is the plywood the large advertising hoardings use. We bought some scrap a while ago which had been used for advertising hoardings for years, judging by the paper build up, and it was still in great shape structurally. Larger timber merchants stock it or should be able to get hold of it.
Al