There is a distinct possibility on a hot day to open the car's trunk to find grocery foods melted in the heat. The trunk remains one of the most cool places of the car, but without proper insulation and packing tips, ice cream can thaw, meat can brown, and ice turns to water. With a little tips to keep in the cold and keep out the heat, you can convert your trunk into a large ice chest.
Rinse towels and dish rags with water. Place towels in the freezer to chill. Once frozen they will be slightly malleable and very cold. Fill large freezer bags about half way or more with water and freeze. These will be your cooler "chillers," which will lower the ambient temperature somewhat.
Place two Styrofoam ice chests in your car trunk. Although NOVA online suggests placing food in the air conditioned section of your car, Styrofoam keeps temperature very well.
Wrap each individual frozen item in newspaper so that it is carefully covered and surrounded, leaving no corners exposed. Newspaper is a great insulator and keeps each item in its own system. As one item thaws it will draw cold from other items, promoting early thawing. Newspaper halts that.
Place two freezer bags on the bottom of each cooler to form a chilled floor. Place each item so that they stack cleanly with little empty space. Use frozen rags to fill empty space.
Stack food until nearly full and place large frozen towels over all the food. Close the lids securely making sure no air escapes. Food should last up to 36 hours without showing signs of thawing.