I just stick with using the seat belt to hold thechild seat in. Has never marked my interior. My 2-year-old daughter's seat is in front, strapped securely in place, no drama.

My newborn's infant carrier (rear facing) in the rear is also held in with the seatbelt. The infant carrier comes out of the car regularly, and I can now secure it with the belt in less than a minute. I've seen some cars with the ISOFIX used, and it does depress the leather.