After running this thing a few times, I noticed that it kept turning off. My best guess is that there was some kind of resettable thermal fuse somewhere near the battery.

I also noticed that I just wasn't getting appropriate battery life. 

I've purchased a Heltec t114 now, no screen or GPS and have had the unit running continuously for 3 days in overcast conditions (with westerly sun, 1/2 day of solar) and in with the solar panel in vertical orientation at home and think I've for the right setup for a solar node I can release into the wild in Newcastle, Australia.

I noticed that the case was ever so slightly too narrow to accommodate the t114, so I've released a V3 of the design which is 3mm wider.

I was also getting delayed trying to be able to print the case in ASA, but decided to just use PETG instead which will give me better heat resistance than the PLA in the prototypes.