If I wanted to do a simple back, it would be a framed fabric piece. For a 53"x53" I would start with a 44 1/2"x 44 1/2" fabric. I would sew a 4 3/4" strip of complimentary or same color fabric to each side then the same size to top and bottom. With the 1/4" seam allowance accounted for, my back would then measure 53"x 53".
53-44.5=8.5 8.5/2=4.25 4.25+.5=4.75
Is this helpful? :)