Roseanna, great job, what a beautiful quilt :)
Here's another suggestion for backing:
http://www.alwaysquilting.com.au/the-art-back.htm
There is a diagonal method that's pretty cool - here is a calculator and nice graphic description on how to do it:
http://multi-patch.com/html/diagonal...calculator.php
OR - purchase some of the wide backing material - lots of stores have the 108" wide fabric!
Enjoy :)