Perhaps a thermoelectric cooler would do the job. Some can both cool and heat. You would have to find one with a flexible thermastat to get the temperature right. If it is connected to a DC outlet that only powers up when the car is running, then it will work to either warm or cool the stored food until the car is parked. At that point it will then insulate it from the outside until it is powered up again.

This doesn't really help James since he doesn't want the bulk, but it seemed like a noteworthy idea.