Last couple of years we have taken our backup car seat (Britax Ranger) it uses the car seat belt
We put it in a Slazenger Cricket bag along with other bits and piecs (nappies, soft things etc..) Firts year when he was three months old, we put in his three wheeler pram as well.
Always gets there fine, plus the Britax seat is a lot lighter than his Bebe Confort normal car seat!