I find that quite often when powering stuff from batteries in environmental conditions where the temperature vary, I end up having over / under voltage issues. My solution then is usually to resort to some Pololu power converters, such as:
https://www.pololu.com/product/2122
https://www.pololu.com/product/2123
https://www.pololu.com/product/2561
https://www.pololu.com/product/2891
I love Pololu, they make great products, and I am very happy to support them, but it is annoying to need to multiply orders across breakouts produces. Could you resell some of their boards, and / or design your own converters? I see that you had some such converters listed in the past, but that most of them have been depreciated. Would be very useful to have some back! 