Just yesterday I pieced a backing only 58" square, it ended up with 7 seams but that won't be a problem, all from the same fabric but I had weird chunks of it. Sometimes piecing the backs takes almost as much time as the fronts, but I have been storing some of these gifted fabrics for years so am trying to enjoy the whole process, and remember it's not a race
Hope you find a solution that works for you.