When it’s familiar with it, use the door
When your pet already enters the transporter, plays inside or sleeps there as if it was another piece of furniture of the house, you should try to close the door. First, install the door and repeat the process of familiarizing your pet with it. Then, try to close it giving it treats and staying close. Don’t leave it inside locked out just yet. Little by little, try going further and further away, trying not to cause stress to your furry friend. Do this for a couple of days for 5 minutes, staying close. Then, go to a different room. Increase the time gradually so that your pet gets used to the transporter.