You can add 2 switches and connect one to usb cable and second to batteries then make hole for those swithes rotate one upside down, glue them both into that hole then glue them together with piece of plastic and now you can switch easily between batteries and usb.
You can add 2 switches and connect one to usb cable and second to batteries then make hole for those swithes rotate one upside down, glue them both into that hole then glue them together with piece of plastic and now you can switch easily between batteries and usb.