From Etsy:
We've made a pattern of our unique car seat blanket (canopy)! Step by step instructions with color photos.
Our car seat canopies drape over the entire car seat, attach at the handles and tuck in at the back with elastic. It's a cute way to keep baby warm, protect him/her from the weather (and germs), disguise an older car seat, and to keep pesky admirers from waking up your sleeping baby.
Each of our car seat canopies is gathered on one end so that it "hugs" the car seat and doesn't blow up in the wind. The corners are finished so that they don't drag on the ground. Now you can use your favorite fabric and make one for yourself!