Nikken Kenko Naturest® Custom Pillowcase. The pillowcase is an ancillary product to the pillow. Complete your Kenko Sleep System with the Naturest Custom Pillowcase. Designed to fit standard-size pillows and the Naturest Custom Pillow, special fibers comprising the Custom Pillowcase provides warm or cool temperature regulation that’s just right. The Naturest Custom Pillowcase is finished in the same pattern as used in Kenko Dream and Kenko Dream® Light Comforters, for a stylish matched look.