Have you tried the pool noodle method of rolling up each layer of the quilt individually before basting? I do this with baby quilts, and it helps keep the backing straight because you can see exactly how it's rolling up on the noodle. Then, as you unroll and glue baste, you can be confident that the backing is pretty much in the correct orientation. If your quilt is larger than the length of one pool noodle, you can join two or three of them end to end with a piece of dowel or PVC pipe from the hardware store. There are a lot of good videos on YouTube about this method.