Battery compartment improvement

The [original PIKON design](https://www.kevsrobots.com/blog/pikon-camera.html) calls for a very specific battery pack, the [NanoWave 3 5000mAh USB-C & A Power Bank](https://shop.pimoroni.com/products/nanowave-3-5000mah-usb-c-a-power-bank?variant=39701956558931).

That battery pack seems to only be available from British resellers, as I haven't found it on US Amazon or any other US-based seller.

And using a battery pack like that also requires an awkward USB-C to USB-C cable that sticks out the top of the camera.

I would like to see if there's an easier way to non-destructively tie an 18650 battery cell into a charge/5V power circuit, and then inject power through that circuit into the Pi's 5V rail.

This circuit would ideally fit in the grip alongside the battery (with the battery and PCB fitting into a channel that is printed into the case itself). I haven't looked too much into 18650 circuits for the Pi, but I know many exist as HATs—that wouldn't work in this build, it would need to be separate and built into the grip area.