This is the best carrier ever. (I've tried 3 others).   It has lots of mesh to allow airflow.  Believe me,  under the seat placement has a higher temp.  Even though my dog has to sit up while I am walking,  It is much more comfortable for him to stretch out when he is under the seat.  I can open the zipper and tether him and he is less anxious when there is no barrier between us.  This is for small dogs.  The carrier can flex somewhat to fit under the seat. Many people have stopped me to get the name of this carrier and to ask me where I purchased it.