Here is the case designed in CAD. You can see a smaller internal box and 2 cutouts in fig.1. The smaller box houses the Raspberry Pi, while the cutouts hold both sensors (see parts list for more info). The outer section will hold the battery and wires. The lid fits on top of the case with enough space to allow ventilation.

Download Case Body | Download Case Lid | Download Full Assembly

The new system would be connected to this app which would tell you the current temperature, battery percentage, etc. This app would allow you to manage multiple HeatSense devices for other cars.

The first design used cardboard to create the case. This is to get a rough design to help build a better second version.

This next design will be made from metal. This is to help with heat dissipation and physical protection.

Instead of having an app where you can connect, since many people won't take the time to install and configure it, why not integrate this into all cars? If temperature goes above a certain threshold, the system should turn on the AC to a lower temperature to cool the interior down. A passive alternative could be to use dark-tinted glass to absorb the light and slow the rate of rising temperature.