Have seen a novel solution to this challenge, although it may not appeal to all - particularly if you require a bulk number of units.
1. Find a small personal or asset tracker, that is very feature rich (3.7V, A/GPS performance, Wi-Fi tracking, 3-axis accelerometer, sensors, light, temperature, etc.).
2. Pull it apart!
3. Source a Qi charger kit (receiver and transmitter), a large capacity Li-ion pack (10-20Ah, in parallel if required?), 2 large neodymium magnets, external GPS antenna (if supported by your tracker).
4. Assemble Qi receiver into Li-ion pack, supply power & SIM to the tracker, place magnets and ext GPS antenna in a row - and - heat shrink it all together - similar to this : https://goo.gl/images/M92Qod
Set and forget!