I quite like the idea of
http://www.samplerbox.org/article/howto ... iwith18650
Drop an industry recognised 18650 in and recharge the device like a phone.
Of course you could just glue a single battery power bank on but the USB cable is thicker and more cumbersome.
Disclaimer:
Don't buy cheap unbranded batteries.
3400mAh is the current recognised maximum for lithium cobalt batteries. You want button top not flat top too. The decent ones have overcharge, surge and short protection (and don't fit in vape devices so you need to ensure your enclosure is for the correct type if going down this route).
The power banks for phones all contain this type.
You can generally harvest these out of old laptop battery packs but for safety and reliability I would recommend buying new.
The ones people buy for vape devices are flat topped lithium magnesium. They tend to 2500 mAh max but have a much higher current discharge rating that is not needed for powering a pi/torch/laptop. They have no safety circuitry.
If there is a better card for use of this battery let me know (maybe one with a charge indicator) but the standing battery advice follows.