refresh my memory, please:
What are the measurements of the quilts you are working with?
I have started making pieced backings and I have seen a LOT of quilt backings that have quilt blocks in them ...
The biggest thing to avoid is: A seam down the middle or across the middle of the quilt, reason being: most people fold a quilt up and the first thing they do is fold it in half, either lengthwise or crosswise, so that seam takes a lot of activity.
If you need more encouragement about pieced backings, I will be happy to share my journey from single fabric to using what I have to make a nice product. <wave>