You are quilting on a rack and quilting fairly densely, you don't even need to whip stitch it. Just cut another piece of batting and butt it up against your short piece. If the cut isn't exact you can overlap it a tiny bit. The excess in the overlap won't even be noticeable with your fairly dense swirls.
I have not shorted myself but have done this with two scrap pieces of batting and once I loaded W&N upside down and started getting pokies so I just cut it and flipped it over. You can't even tell!