After 20 hours, I returned to the workshop to discover the test balloon had completely deflated (as I expected), so I set to work trying to find out why. First, I inflated the balloon again (this time using a hand-operated bicycle pump for precision), then I sprayed soapy water all over the wing root area, all the seams and fittings, and the taped balloon connection. The problem became apparent pretty quickly. The vent fitting, the taped balloon connection, AND the fuel fitting were all bubbling like crazy.

I cut off and re-flared the tube, but the vent fitting still leaked. So I used Fuel Lube on the threads of both AN fittings, and that sealed them both perfectly.

Next, I tried two more techniques to attach the balloon to the aluminum tube before I found a method that worked without leaking: I cut off the bottom "lip" of the balloon, connected the remaining balloon with a very tight O-ring, and wrapped it all tightly with duct tape.

After inflating the balloon again and spraying more soapy water, there were no more bubbles anywhere, so I left the balloon inflated and moved on to other projects. Two hours later, the balloon was still fully inflated, so I think we're on the right track. I'll check it again in the morning.