Hi,
I don't think you are supposed to attach it to a carseat. Maybe just any size small quilt that will drape over the child and lay loose. I used smaller 40x40 quilts and up. It crossed my mind you might be thinking of a stroller quilt. Here is a link to one that can be tied for a stroller.
http://littlebirdiesecrets.blogspot....ler-quilt.html