I stumbled across a 3/8 tubing nut and sleeve in one of the parts bags the other day (it was on the bench..) and that gave me an idea on how to fix my leaky balloon problem. I pulled the leaking balloon off the fuel tank input bulkhead fitting. I flared a scrap of 3/8 tubing and added the nut and sleeve. I then taped the balloon on with electrical tape and added a couple of tie wraps. I tightened it on the fitting and put some air in the tank. This is much easier (and hopefully less leak prone) then trying to attach the balloon straight to the fitting.
Balloon attached to length of 3/8 tubing this time