I use mostly hi-loft polyester batting. A few days ago I needed to add a thin strip of batting to get the length I needed. I put Elmer's washable school glue on wrong side of backing laid strip down, put a strip of glue on top of batting and laid edge of quilt on. I use large safety pins and pinned across the joining pieces, laid it flat and let dry overnight.
Next day I could not tell where it had been glued. Quilted up like the rest, no hard places.