It's worth noting that this kind of thing works great at a smaller scale. Adafruit has been marketing bare PCBs for sew-on applications for years.

On an even smaller scale: I contributed to a beginner soldering class by designing this simple board shaped like the high school's logo: https://postimg.cc/ftwtqHFn (for the record, shorts across the contacts in the exposed metal area are harmless. The transistor never saturates in this circuit)

The key to Adafruit's arduinos-but-for-cosplay, and my keychain photocell thingy is that neither demands two(!) 18650s worth of power. There just isn't a significant hazard in the first place.