Someone already mentioned taping the fuel caps. I put the fuel caps inside a glove and then locked them in the openings.
Also, I found using a water manometer to be much easier than the balloon. The balloon will contract and expand with temperature changes, which you have to account for.
With the water column, you mark the water level when you start the test, and note the ambient temperature. You then wait until the next day, when the temperature is the same as when you started the test, and check that the water level is at the mark you drew the day before. If you are lucky, you can come back on day #3 and smile as the water is again at the same level
Here are some (mediocre) pictures:
http://rv7a.fabs-designs.com/2009/09...ing-the-tanks/
Good luck!
Luis