You might want to try a different technique next time that does not require the slip-stitching of long seams in back. I can't remember the quilter's name, but she wrote a book that included this technique that she uses for large quilts. I'm not sure I can do an adequate job of explaining it here, but I'll try.
The top, batting and backing are layered as usual. Baste the middle only.
Peel the right one-third of the top and backing to the left, exposing the right one-third of the batting. Using a rotary cutter, she cut the batting in a big S-pattern down its length with the width of the S being about 6 inches. Remove the right piece (probably want to label it top/bottom/front). Do the same with the left one-third of the batting.
What this does is allow you to quilt the middle one-third of the quilt first without the bulk of the right and left batting pieces. Just be sure to leave about 6 inches free of quilting on each side of the middle piece of batting. There is much less bulk to handle while quilting this way, and just the top and backing fabrics to roll up or fold under the machine arm.
Re-attach one side of the batting to the middle part, again peeling back the top and backing so you can work just on the batting. Use a tailor-tack type large hand stitch to secure the right one-third of the batting to the middle one-third of teh batting. (The stitch is like the one Sharon Schamber uses to baste quilts on her Youtube video.) The S-shape keeps the join from separating later and forming a crease in the quilt, plus it helps you position it back in its original place. Quilt that one-third.
Reattach the last one-third of batting and quilt that one-third.
The piecing of the batting will be invisible, there will be no hand-stitching required for seams on the back, and all of your quilting will look as if it was done on the whole quilt.
Another tip about batting. Many battings have more stretch going one way than crosswise, same as fabric. The best way to layer a batting is to have the stretchy grain running up and down the length of the quilt, with the less stretchy crosswise grain running left and right.