What about after you've quilted the fabric (quilt sandwich, but before you sew into the bag), you make a long pocket (end to end of bag, but only as deep as you want the pocked - small or medium - making the pocket with french seams top and bottom - leaving edges open for turning). You would be able to sew pocked in place when doing your side seams, so you would only have to tack the pocket in place by hand so as to not take away from your quilting. Just a thought. Good luck, I'm sure someone else will have a better idea.