Here's a cracking link to how to build your own battery pack to fit in a pack of cards box. I particularly like the use of UHU for the construction - quality or what?
This should not only cure your battery problem, but add some interest to your flight when they discover all those wires and batteries concealed to look like playing cards.
If are you flying in the south in the Northern hemisphere or north in the Southern hemisphere, they might last a bit longer due to the magnetic flux resonance...