What about a ruggedized phone like one of these for example?
"and a non-removable 5000mAh Li-Ion battery with an outstanding 38h talk time and up to 44 days on stand-by."
Wow. Maybe I would get one just for that!!
I have a Ulefone Armor 3W. 10000mAh. It's a massive tank but not having to charge is god damn amazing. This brand may also be applicable to the OP>