I personally designed and 3D printed a case to hold 4 rechargeable batteries, so I could charge them with 5 Volts from a USB cable.

Fun Fact: this ruins the batteries. Gave up on designing myself and downloaded a design for a battery-adapter (plastic shell + 1 screw that makes small battery fit in big devices). My stockpile of small batteries then lasted me 2 months before I finally bought a charger and new rechargeables.