If you want a back with no seams then you need to buy extra width fabric (I think it comes in 120" ... I don't remember!).
You could also piece the back with two pieces of fabric sideways. If it's a large print I like to match the print, if it's a small print it doesn't bother me.
Or ... instead of two strips down each side ... run a single strip down the middle. Of course when you do this you need to center it so it doesn't look wonky.
Or ... piece the whole thing from large scraps of the fabric from the top. I've seen lots of lovely backs that were just large squares or rectangles pieced together.