24 hours later, and I'm ready to test the video penetrator. I've attached the endcap to the rest of the seal pieces, and inserted them into the 4" tube.

This is then upended and placed into an old gallons milk contains (with the top cut off) which has been filled with water, such that the penetrator is completely covered. No immediate leaks, so now I just have to waiting see. Ideally I'd later do a pressure test, but not sure how to do that yet.