Half a tank of Coumarin Pipe later, the self-trimmed O-rings have settled in and tightened a tad more on the Diver, so the tank doesn't slide easily any longer. I really need to tug hard to make the tank slide at all. Perfect! (I'll shut up now)
I took the end cap off the Diver, slid the carto driver into the side of the Diver body with the threads, and then used it to push the Diver body through the bottom cap. It went through with some small amount of force, BUT it also sliced off a thin sliver off the inner circumference of each of the two O-rings. Same with the top cap. They are still intact, just slightly wider inner circumference. And the Diver slides through both caps now. Not sure if it will be leak proof now, but I'll test it out tonight and see.
Seems OK so far, but the tank does slide off the Diver a tad easily - but it doesn't let any liquid out, so the O-rings have a good seal.