No test yet. Have bad cells to do the test with but not yet time when I had energy/etc. Same as with most of my projects.
Should be no issue with water getting in here in Phoenix, at least.
But up where you are, you could do as dogman suggested, and just put something like scotch tape over a hole (or holes) drilled into the casing. If any pressure builds up, it should pop the tape open and let it vent.
You'd have to test the theory, of course, but you can probably do that with baking soda and water in a few water bottles rather than a burning pack. Fill the bottles with sufficient water, then the baking soda can be dropped in maybe inside a water-soluble container or something. Or use cola and a mentos stuck in the lid.
YOu don't need the lid screwed on tight, don't need an explosion, just rapid pressure build up as in a pouch fire.
I thought about doing that for a test, but I'd rather do it with actual cells since I have them.
(and who can resist a good fire?)
Anyway, the way I have mine setup it should vent fine, though if the gases are hot enough it'd probably light the wirign on fire. But it should also keep enough atmospheric oxygen out of the case (since it's under positive pressure during venting) to prevent the pack from burning from what might otherwise be a simple venting. That's what I really want to test with the bad cell overload test.