Caroline,
I also think muslin is a good choice! For me it is always very hard to get fabrics 108" wide in the quality or color I want to have. Therefore I mostly take the "normal" wide fabric (44-45") and sew several length together. When the seams are crossed by many quilting lines you almost won't see it anymore.
One important thing: never do a seam in the middle of the top - a full length in the middle is much better.