The problem with any film forming finish under those conditions is that they simply won’t hold up. That is why they aren’t used on decks. I would be looking at an oil based material that will prevent water caused degradation. I like Thompson’s Waterseal Timber Oil. Another recommendation I have is to treat all of the wood prior to assembly in order to make sure all of the surfaces are treated. Water finds its way into the joints and mating surfaces and tends to stay there longer than on the open surfaces. Another hint is to embed exterior grade metal fasteners (if used) in the wood using countersinks, and plugs, to prevent them from condensing moisture. Use water resistant glue ( if used at all) and avoid cross grain gluing which is more prone to temperature and moisture failure.