just a suggestion...
Look at regular "dorm cube" or "dorm fridge" and a cheap MSW inverter... fridge will run while the car runs, and hold the cold for a long while when the car isn't running. install the inverter under the hood somewhere and run a 120v extension cord to the fridge - easier and cheaper than running heavy gauge DC cables.
12v doesn't provide a lot of torque to run a compressor, you need huge wires. So a 12v compressor is gonna be expensive (brushless DC motor with rare earth magnets) or ineffective (that's why RV use ammonia instead and a propane flame.)