USB-rechargeable lights/hosts for modding

I'm looking to make some lights for gifts. I'd prefer them to be USB rechargeable, if possible, but I'm not interested in leaving them stock. I'm looking for the following criteria in a light:

  • USB rechargeable
  • Moddable (let's go with, "easier than a ZebraLight" here)
  • Smaller than 18650 battery size
    • Ideally 18350, 14500, 16340, or 10440, but other small cells are fine.
  • Charging is not integrated into the driver
    • Otherwise, the driver would have to be moddable itself - with a good UI (L->M->H) and options for output modding (even if its a sense resistor swap)
  • Bonus if it comes in something other than black (now I'm really dreaming right)

So far, the closest I've come up with is an Eagle Eyes X2R with Convoy 18350 tube (and potentially tail) in another color. The X2R head of course only coming in black and gray.

Any ideas? I'm also up for advice on DIY-ing charge circuits into lights. The circuits themselves seem to be cheap and small, but I'm not sure of a good way to integrate them.