Comment What's your budget (Score 1) 202
Are you trying to cheap this off or do it right?
If you're going cheap, put it in an ammo box. Force through it with a duct fan, I'd weld flanges onto the box (since I've got my MIG up and running now) to attach the up and down pipes. This solves your air circulation problems to the point that you might not even need fans in the box. Ammunition cases have rubber seals. You may have to inspect the boxes carefully to find one with a good seal. You'll still need a drain as well, and it should have a long hose attached to prevent spray, and a restrictor to prevent bugs.
If you can do anything you want, water cool it and make a water block to go on the outside of a truly well-sealed box. Monitor the temps in the box and run seti, folding@home etc for heat if necessary.
Or, how about a waterproof enclosure which can't handle direct spray inside of a bigger box with drains which will deflect it? That's how cars work. It's remarkably effective