I have always said if I went on a long car trip with the kiddos again I would get one of these:
Pet Carrier they also have a single version if you think that is too big. I like mine to be fully contained in the car I think it is safest, prevents them from getting thrown from the vehicle in an accident and also makes it easiest for emergency personnel to transport them should an accident occur where you were not able to help. These provide that while still giving them all the room to move around.