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Do not buy a foam pillow insert that has a height dimension as well. You want something that looks like the pillow you'd put on your bed or couch. A 16 or 18 in (41 or 46 cm) pillow will work the best, but you can use any size you want. This method may work for round pillow shapes as well. Take 1 of your panels, and turn it so that the right side is facing up. Take a zipper that is 3 to 4 inches (7.6 to 10.2 cm) shorter than the panel, and place it face-down on top of the fabric. Mark the ends of the zipper with a fabric pen or sewing pins, then set the zipper aside.  Do not pin the zipper in. You need to sew and press the seams between the panels first. An invisible garment zipper will work the best, but you can use a non-separating garment zipper too. If you cut the panels to the exact dimensions as the pillow, be prepared for a snug fit. Once you have the pillow all the way in, zip the zipper shut.
Buy a pillow insert in your desired size. Mark the length of your zipper along the bottom edge of your panel. Insert the pillow and close the zipper.