Well, at least I *think* there are no leaks in the left tank.
All the leaking I could detect was through the fuel cap, but as long as that wasn’t sealed, there was no way to determine whether other pathways were still open.
I finally figured out how to seal up the filler cap (used transparent 2″ wide shipping tape, because it had much better adhesion that duct tape, and any air pathways and/or pockets were easier to see through the transparent tape).
After 3 hours, the balloon is still inflated! I’ll check back in the AM to see if it’s still holding.
UPDATE: 12 hours, balloon still big!
You can see the clear package tape on the tank skin…the folds and bumps are on the top layer (there are at least three layers of tape covering maybe 12 sq in of surface area above the cap).