cad files for rear case/battery upgrade

[wibble]

I think I misunderstood what you're doing. If you're needing the NTC I guess you're looking to use the normal battery terminals and replace the usual battery entirely, rather than adding a second battery as I thought. You should check out what the charge controller is doing as your battery may have different voltage characteristics to the stock one (max charge voltage, minimum voltage etc.) I'm not familiar with that bit, but Megous' site might be a good starting point:
https://xnux.eu/devices/pine64-pinephone.html