I used the same method as SpeedBump with 0.5mm copper strips. Any thinner & it will breaks easily with all the vibration. Just lightly sand the battery terminals & strips & solder them together. However, do be cautious not to place the soldering iron on the terminal for too long as it will overheat the battery, damaging it or causing it to burst. Prior to soldering, C.A. the batteries together neatly in the correct polarity first & measure approxiately how long the strip you need. After you are done, wrap the battery pack up in cloth tape to protect it.
If you have problem fitting the pack(should be, as the box is not meant for 'AA' battery) into the box, below are some pics to give you some ideas.
The aluminium cover is soft enough for some copper-tooling work to get the shape you desired. I've even added some mounting posts & screw the cover on for peace of mind. I experienced the cover deciding to part with the box midway through my final & ejected itself together with the battery pack.
You should hear the beautiful sound 'klink, klink, klink' & see how shiny the cover is when it flies up the air after popping out & was caught by the rear wheel.