Your quilt tops are beautiful. I'm in the same situation. I have lots of fabric and need to choose patterns to use up some so I can buy more! For backs, I've been taking fat quarts and cutting them in half (into fat eighths) and sewing them into a brick pattern for the backs. I like that it makes the quilts reversible, and uses up some of my smaller pieces of fabric. One recent quilt I cut the pieces into 12x18 because I had larger pieces. I love a pieced back.