Once I had PCBs it was time to make a pack.  I already had nickel strips and a spot welder from previous efforts, so that made things easier.  I also got flexible silicone wire, JST VH terminals and housings and a crimper that works for JST VH.

First I soldered the wires and nickel strips on to the PCB:

Then I spot welded the nickel strip to two 18650 LiFePO4 cells:

As you can see, the components sit in the gap due to the radius of the two cylindrical cells to get a compact assembly that only takes up the space of two 18650 cells:

One of the parts you see at the end is an NTC thermistor.  Since it's supposed to measure the temperature of the cells, the air gap is not ideal (although once the pack is wrapped, the air being trapped in the pack should improve things versus open air).  I asked a manufacturer in China I'm talking to about mass producing these about filling the gap with some thermally conductive compound and they said it was possible.

A real battery pack would at this point be covered with foam end caps and wrapped in heat shrink, but since these are prototypes and I will likely be messing with them, I decided to leave that off for now.