I usually plan (if I plan at all) to have extra blocks or strips that match the top just to add to the back. If you don't have extra from the top you can add your strips with extra backing fabric. Buy 4 yards of the backing. Cut 2 12" strips and sew together end to end. Then sew those strips onto the side of the 3 1/4 yard piece. Don't forget to cut your selvages off so they are not part of your seams. It won't be as pretty as MTS method, but it will work. Whe I need to piece the back I try to do something similar to MTS, but it's not always feasible.