If possible I would bed them in fairly deep straw and close any vents from the inside. Open sides shouldn;t be an issue as long as they are out of the direct wind. I would rather do it in the cold of winter then the heat of summer
Dry and a full belly, I wouldn't worry about it at all.. I have an open stock rack and hauled lots of cows in colder temps than that for about a 4 hour drive..