I just made a back for a large quilt and used large rectangles in a brick pattern. By large, I mean almost 1/2 yd each rectangle. I cut them all down to 16-1/2 x 32-1/2. You could do similar between the pieces of black. This pic is a sample of how it looked as I laid out the colors. I don't have a finished one to show you yet (it's on the way back from being quilted by Charisma). You can see it on her blog. I love how it finished up.
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