If your car is big enough for a crate that is big enough for a litter box, that will help.
We are anticipating a move that is 8 hours away in a few months. We have eight cats to transport. We have talked to our vet, who has told us he can give them each a shot that will put them into a light sleep, which will help them keep calm during the journey.
Perhaps you can talk to your vet about something like that? Good luck!