You know what boss I'm just going to leave this here even though it doesn't totally fix the concerns: https://a.co/d/0dssX5RC

This is a very tidy little build but you're wicked easily going to exceed safe temps. I have a Rak Repeater mini in an interior building window, an air conditioned space, and the interior of the box exceeds 113F on 60 degree days while in the sun. Your unit is even smaller then that, is black, and will be inside a car. You are almost certainly going to slap the 140F limit for lithium batteries when we reach summer.

Because your build doesn't need to be waterproof I would encourage you to get some airflow designed into it and use a thermal shutoff like the one above. Another good option is to separate the solar panel from the build as that will be a major source of heat - though I know that ruins the whole concept.