Your cat should be able to stand up and turn around. Having front and top openings make it easier to get in and out. It is best to have a top half that is easily removable.
Make the carrier a familiar place
Leave it out in a room where your cat spends a lot of time. Remove the door or secure it to make sure it stays open. Put a soft towel or blanket inside. Cover the carrier with a blanket to make it a cozy cave to hide in. Place treats or toys inside on a regular basis to make it a fun place to explore!
From home to car
Cover the carrier with a blanket. Don’t tip or bump the carrier as you walk with it – hold the bottom. Warm up the car in winter and bring the car as close to the door of your home as possible – cats don’t like the cold!
In the car
Don’t feed a meal before travelling (treats in the carrier are OK). Place the carrier on the floor where it can’t move around. If the carrier is on the seat place a towel under one side to keep it horizontal and secure it with a seat belt.
Arriving at the animal hospital
Place the carrier on a seat or table (cats like to be high up). If your cat is nervous ask to be placed directly into an exam room. Once you are in the exam room open the carrier and allow your cat out to explore (bring some favorite treats!)
Feliway spray (a synthetic feline pheromone) can be used in the carrier at least 30 minutes prior to travel to help calm your cat.
For more tips go to http://catfriendly.com/be-a-cat-friendly-caregiver/getting-cat-veterinarian/