When we picked up our puppy I drove and my wife held it in her lap and we put a blanket and towels on the floor of the car in between her feet for it to sleep on. Also that way if it has an accident or gets sick it can get on the towel. I would probably expect to stop and let the puppy out pretty often at that age they tend to need to go once an hour if not more.
Athena -GSD- born 12/13/12
Luna -Shepherd Mix- born 3/20/99