I'm looking all over the place for one and can't find it. Plenty of them around to sew or crochet. I can crochet but would rather knit. Here's some patterns that may give you ideas. The first are simple. It would just be a small knitted blanket with loops which you could either pick up the stitches for on the top edges or knit separately and sew on.
Another thing I would suggest would be to knit a small blanket which would fit over the car seat opening and make a casing on both the top and bottom edges of the blanket. You would run a piece of elastic through the casing. This would be knit in stockinette stitch. You would knit about one inch, do one row of k1,yo all across. Keep going in the stockinette stitch or your pattern of choice for the length of the carrier, k1,yo on one row, knit in stockinette for one inch. you will fold the hem along the yo rows. Sew the hems and run a piece of elastic through them. This will hug the carrier on the top and bottom edges so the blanket can't come off.