Here are a few ideas.
If you have a large piece, you could use that as a center back and simply add wide border strips to top and bottom.
Again, if you have a large piece, you can slash it diagonally and sew a wide strip of contrasting fabric in-between.
You could cut 24 individual pieces 12-1/2 inches square and piece them together 4 blocks wide by 6 blocks long. If you need a little extra width, add a border strip to each side.
With pieced backs, my philosophy is to keep pieces large and minimize the number of seams. Also, consider pressing seams on the back open so you don't have areas where you will have to quilt over two crossing seams pressed in the same direction. (Disregard if you are planning to tie instead of quilt.)