Flannel does like to act up sometimes. The last flannel back I used was with Hobbs 80/20 fusible batt and I did get a 105X105 machine quilted without any wrinkles but I had to re-iron the back a couple of times as I quilted. I have not used the Elmer's washable glue on a big quilt so I don't know how that works. I think either the glue or 505 basting spray would probably be your best bet.