There are several herbal remedies available to keep cats calm during travel. You may want to consider one of them. I just used Google with this:

cat calm remedy travel

Several brands popped up. Of course, you could get something from the vet as well. I have heard great things about some of these herbal products, so if you can find the time, look into them!

Rescue Remedy is quite popular, as is Bach's Flowers products.