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.... I am in the final stages of testing a privately designed, 3D printed Delrin battery hatch cover and a baked Aqualung I550 battery hatch cover that appear to now fit into my two....
In any such
commercial venture you better damn well test in real conditions. Any failures post-release will put you (and anyone that appears to be you) right out of business. This will occur because of really bad PR, possibly lawsuits.
Subjecting things to stress and past design parameters is often referred to as
destructive testing and that can sometimes cost money. The cool thing is that after you ruin one "housing", it now becomes the last and only one you'll need to buy. If it floods, dry it out completely and put a small spansule inside, maybe a Benadryl. If it melts, this is an indication of another failure.
You should also establish an ongoing periodic sample test schedule of production. It's just the accepted process.
I like the hanging it over the side on a rope method as prior post.
I'm the guy that advised MagLite in 1984 that their packaging stating "waterproof" might better state: "Waterproof, if in the OFF position". In 1998 or so I ruined a whole lot of Inova UV flashlights that were also "waterproof". I found out that Spyderco Dive Tools became exceptionally dangerous after just one cycle of salt water immersion. I get paid to wreck $#i+
While you're at it, you might want to
Google "waterproof iP standards rating".
Details absolutely do matter.