Jerrie,
I've made 4 of these rag quilts now. I love these, they go together so fast. I've made them all with cotton fabric on the front and back. I usually use flannel in between but on one baby rag quilt, I used batting. It was alot lighter than flannel. If you use flannel, just cut your blocks the same size as your cotton blocks, if you use batting, be sure to cut them 1 inch smaller. You can use any fabric you'd like. There are a few on here who have made jean rag quilts, they are very nice but very heavy, I'm sure. I don't know if you were planning on quilting it or not. But, if you use flannel, you don't have to quilt your blocks, however, if you use batting, it would be best that you do. Be sure to post pictures when you complete it, we'd love to see it. :)