Boy, do I have lots of links for people today. I searched the net some time ago for these things. Never did make one, but I have seen others that have and the person that used them thought they were great. Just wish they had them when mine was little!
http://www.ida.net/users/stace/sling.html
Here is a place for a pattern.
http://www.mammasmilk.com/ebooks/