I carried a four tier cake on a five hour trip to Virginia Beach at the end of June in 90+ degree weather. I ran the air conditioning wide open all the way. It had to be assembled when I arrived. It was for my son's wedding and went off without any trouble. I found some stackable plastic storage containers with lids (from Big Lots) that had grooves in the top where they could be stacked on top of each other without worrying about them sliding around. We even layed our formal clothes across the top of them to help keep the heat off of them.
I did this a couple of years ago. I put all four tiers of the wedding cake plus a groom's cake in separate boxes in the trunk of the car and carried them 5 hours for my aunt's wedding. Now I have a mini-van and it would make it so much easier. The wedding cake was in buttercream and the groom's cake was in chocolate fondant. Everything turned out just fine - but I was a nervous wreck. I made sure to carry extra supplies to repair anything that might go wrong but nothing did. It all turned out great and my aunt and new uncle are estatic. They were teenage sweethearts who were reunited after both of their spouses passed away. They were in 78 and 80! It was a great day!