30-40 gallons! That's a lot of cherries...think 8 five gallon buckets. I would put as many as possible inside the car, but if you have that many, you'll probably need to use the trunk as well.
Michael's idea about the dry ice is a good one.
You could also put a layer of ice in the bottom of a large cooler, then line the cooler with a garbage bag. Fill the bag with cherries and close the lid. Or seal the bag and put more ice on top.
You're really not worrying about spoilage here...you just don't want them to cook from the heat in the trunk.
I hope you are going to have help pitting all those cherries! I put up about 10 gallons for pies and toppings one year, and by the time I got done pitting them all, my arms were like sour cherry suckers from all the juice dripping down them. Of course, you won't have to pit the ones you use for jelly.
One thing you can look forward to...your car will smell like cherry pie for several days. I brought home a backseat full of strawberries one time, and the aroma lingered in my car for some time.
We get by with a little help from our friends