As shown in the last photo, as an example, using 18650 Li- Ion cells you can get 16 cells with out cutting any plastic, and looks like this would allow room for battery protection boards, which I recommend. A good source for free batteries are old tool or laptop batteries, these usually have a couple of bad cells and that's what causes the pack to fail. If you need mating connectors, check my other listings.
The item "US Military Radio Battery CASE BB-490/U Add your own battery cells" is in sale since Sunday, April 1, 2018. This item is in the category "Collectibles\Militaria\Surplus\Radios".
The seller is "cardinal-177" and is located in Smithville, Missouri. This item can be shipped to North, South, or Latin America, all countries in Europe, all countries in continental Asia, Australia, South africa, New Zealand, Egypt, Reunion.