It looks really, really good! What size lumber did you use? I like how you sandwiched the screen between the wood. Makes it look very "clean".
How are you going to drain away excess water?
You can seal the wood. As long as you give it the proper amount of time (or longer) to cure it won't be a problem. I think potential wood rot or mold from moisture is more of a concern, which may not be an issue with cedar (I'm not sure, I don't now much about the different wood types).
It looks good though! I think the biggest concern with having a big cage for such a little guy is whether or not they will be able to find their food. I have my 3-month old veiled in a 24x24x48 and he does just fine! I put the crickets (10-15 to start) in an old Cool Whip dish wired to a branch, they stay in long enough for him to snag about half then the others get out and he gets to free range. I very rarely find any left at the end of the day and sometimes they're all gone by noon and he gets an afternoon snack.
I love enclosure pictures! You should definitely post pictures when you get the "furniture" loaded in!