I've been wanting to make one of these for our upcoming baby...so I bought some flannel that I liked and found this tutorial on Make it Do just in time to make it a breeze to make.
It even has curved corners, so it won't drag on the ground :) My favorite part it that it has straps to hold it onto the car seat and holds it off the baby's face. Here's a peek at the underside flannel:
So I readjusted the car seat straps, put on the cover and one more baby thing is ready!!