The bigger pieces I sew together, I have made several battings like this. I mainly use polyester batting and I hand sew them together with a large zigzag or a ladder stitch. I save long skinny pieces ( 3" or wider) for adding to sides or tops or bottoms when I need a bit larger.
Sometimes a pain but I am able to use them and not store them. Smaller pieces I give to my Daughter for the pillows she makes. Sometimes I just throw the skinny pieces out.