You can get something , tablets, from your vet caled Zylkene. Not exactly a sedative but having the effect of making less stressed. Used for travelling, fireworks etc. We had a lot of positive feedback about them at the vets where I worked. I think you have to give them for a few days or more before the stressful event.
Also make sure your carrier is completely escape proof, a cat doing a wall of death round a car may look funny but could cause a major pile up!! I have seen a cat choke to death by getting it's head out & stuck with one of those wicker baskets so don't use those. Sorry but after many years of vet nursing I have so many horror stories & am paranoid about safety!