Well, you could have a soft hide in there, like a wash cloth. Some people will decorate their separate containers a bit and it would be less stressful if they had something to hide in when they are scared. But, a wash cloth or two would lessen the risk of crushing them than, say, a piece of wood. I don't think fake plants would hurt if you wanted to put something in there.

They also need water, but it's a bad idea to leave one in there if it'll be bouncing around. I guess, too, it depends on how long they'll be in the car. If it's only a couple hours, I think they should be okay with nothing. It's really hard to give a straight answer because everything depends on the snake and other circumstances.