Do you use pieced batting? I often use it. I keep leftover pieces of the same brand and loft together. When I have several chunks, I trim to make straight edges, use a long, wide zigzag stitch, simply butt the edges together and sew.
Be careful to make sure both sides are feeding under the needle at the same pace. You don't want to stretch one, it causes puckering. Generally, I only piece for a quilt less than 50". Anything bigger, I use packaged batts or "off the roll".
Frankenbatting is great for baby quilts and small charity quilts.