If you can't change the backing to a vertical layout, you could try a small piece of batting on each end. The width of the batting should not be more than the circumference of the bar; keep the length manageable, 6-8 inches should be enough. The batting should be placed on each side at the bottom of the backing just as you start to roll it. This should provide enough "bulk" for the backing to be flat when you reach the top. Just make sure to remove the batting before quilting the last row to avoid getting it caught in the stitching. I also find that it's sometimes easy to stretch the backing while attaching it, especially if it's pieced. Measure the backing side-to-side in three different places, and use the average. Then centre this measurement and temporarily mark your leader cloth, line up your backing fabric, and attach. If there is too much fullness, the backing may need to be trimmed. Good luck.