Put it in the trunk. It should stay cold enough to keep condensation off it until you get home. Even better...put it in a plastic bag and put it in the trunk. If you have a truck, put it in the bed in a bag in a cardboard box to keep it from rolling around. I wouldn't even think of cleaning it in the field, especially in those temps. And here I thought I was anal about cleaning my gun. You make me feel better.
I use a CO2 discharger to clear my barrel. If I didn't fire the gun that day, I just swab and oil the bore when I get home.