Initial fit was good and the flange appeared to be in place...
...but I noticed a small gap between the skin and the flange when looking at it from the side.
At first I assumed my countersinks weren't deep enough, but a couple of attempts to deepen the countersinks resulted in almost no difference with the gap.
Since the flange is machined to match the final curvature of the tank skin, I figured that the curvature is probably different when the tank isn't attached to the ribs thus my countersink depth is probably fine. Looking at the fit from through the hole itself, it was difficult to even know that a gap existed. The gap will most likely be sealed by the tank sealant I apply here when I rivet the flange to the skin, so I decided to stop worrying about it and move on.
I countersunk the holes in the flange for the other tank and got similar results.
The small clips meant to secure the very end of the vent line needed to be fabricated, so I grabbed some scrap sheet and cut some small strips. I bent them into shape around a drill bit that was roughly the same diameter as the vent line. Some strategically-applied force with my fingers and later with some pliers gave me roughly the shape I needed.
I did my best to make them as round as possible but it was difficult to keep them from getting sharp bends.
Once they're riveted in place they should work well. Their only purpose is to secure the end of the vent line and they'll certainly do that just fine. For extra insurance, I may use a dab of tank sealant between the vent line and the clip just to keep things stable.
