I could spend money, but....

If you want a battery box intended to hold two batteries to run off only one battery (halving the power, the speed of your motor, and, one hopes, the noise), you can buy an expensive widget the size and shape of a battery but with a short through the middle.
Or, you can take a bit of balsa the right size, and wrap it in kitchen foil.
It works. I'm not convinced it gives enough power to drive what it's meant to drive, but the experiment was free.