The 12 volt "remote control" battery shown at right is available
from WalMart at a cost of 2 for $1.77.
find some washers that are just slightly larger than the individual cells.
Solder wires to two of these as shown above. I used black
for the negative end and yellow for the positive.
a lighter or other heat sorce, shrink the whole thing as much as possible.
The tubing also shrinks along its linear dimension which pulls the whole
stack firmly together.