This is the one I made.
I used the basic idea from the pattern and just altered it a bit. The link has a basic and a more custom fitted cover, I did the simple because I wanted it to fit in lots of different carts and high chairs. Also, I did not put any batting in it as was suggested because I wanted it to fold up small enough to fit in my diaper bag or the bag that I haven't made yet to carry it in :). I used 2 layers of home decor fabric so it was still plenty thick without the batting. I will post pics as soon as I remember to take my camera to Walmart to get a picture of it in action! For now, you can find the link here.