Hi mate good luck in the build, I'm in the process of modifying a display cabinet for my young veiled, what species is this viv for? As for wood, you'd be best with ply or MDF and coated with several layers of yacht varnish to waterproof it and also use aquarium sealer on all the joins and cracks , as for the screen you dont have to treat it , I found a staple gun was perfect for securing my screen top and back, to prevent water spraying everywhere ive seen shower curtains being used on the outside although personally I'd go with a camouflage tarp for better looks, one thing I'd say you need well thought out is a drainage set up, for me I'll be using pond liner on the lower 4" of the viv sealed to the walls with a piece of tubing sealed in the centre through the bottom of the viv creating a funnel affect for the water to collect and drain into a bucket, the viv would have to be on a table for this to work but its the best I've came up with for my set up