I have a Solvit too and have my seat belt that goes around the front to secure it in addition to the strap that comes with the car seat. My two fit in the medium one. Murphy is 7 pounds and Mia is 3 pounds. I bought mine at a pet expo so I was able to ask the guy who sold it to me for an extra harness hook which comes in handy with both of mine. I also have no one who rides with me in most cases so it sits in my passenger side seat most times. Murphy and Mia love it - especially nosey little Mia since she just loves to look out the window, Murphy generally lays down and sleeps.
Oh and I have my airbags on their side turned off.
