While the canisters have a listed maximum storage temperature, there’s still a safety margin.
The trunk is better than in the sun on the back seat if the windows are closed. Â But in the foot well of the passenger compartment, on the shady side, with the windows cracked is far cooler than higher up in the passenger compartment.
I like having a soft-sided cooler in the car for many reasons. Â Trader Joe’s has a nice, good-sized one for not much $$.

I use it as an additional reusable shopping bag and put the milk and ice cream in it (in the summer) and the lettuce and cut flowers in it (in the winter). Â It folds up pretty flat when not in use. Â On road trips, I keep dairy and cheese and meats in it for cheaper, healthier, on-the-road food. Â I also put a few pre-made salads in it and a real metal fork to eat them with. Â On week-long road trips, I score ice from hotel ice machines (cutting the spout but not the handle from a gallon milk jug makes a great ice cube carrier), or if my room has a fridge in it, I fill repurposed soda bottles 80% full of water, freezer them on their sides overnight and then use them like (free!) blue ice each day.
I often use it for transporting frozen salmon across state lines, sometimes for 20+ hours in transit.
Simply putting your canisters in a cooler will moderate their temperature. Â The more water in the cooler, the more moderate temperature swings will be. Â The more frozen bottles of water you include, the longer it will stay cool.
Putting the whole cooler in the freezer overnight (we all have 3 chest freezers in Alaska) helps. Â So does wrapping the most critical stuff in towels, bubble-wrap, or Reflectix. Â Sometimes, I’ll skip the soft cooler and just put the vac-packed fillets in a plastic bag, that wrapped in a towel, that in another plastic bag, then a jacket, etc and leave it all in the freezer for 12-24 hours.
Outside of the cooler in your car, spreading your sleeping bag over/around it helps A LOT. Â It turns out – surprise, surprise – that your sleeping bag is quite good at insulating things.