The scientific pursuit of underwater hamster objectivism
Question; Why are you only using dive boxes? Wouldn't one of those plastic storage totes work well? They're fairly cheap, much larger, and many are even transparent. You might be able to reach greater depths by stacking them two or three thick (as most of them are designed to nest, which would conceivably increase strength) at the cost of visibility through the plastic. You might have to modify it somewhat to make access possible and ensure it stays waterproof, but since you're already running overpressure and they're so much cheaper than diveboxes, it seems like a decent trade-off. If you need proof of long-term survivability underwater, I can go buy a couple and huck them in the river (30+ ft. deep) , then provide documentation on YouTube or something.