if you are making the pillowcase from scratch, print the periodic table onto fabric and then add borders until the top is the right size.
if you are decorating a pillowcase that's already made, print the periodic table on fabric and then applique it to the preferred side. frame it using ribbon or some other manly-nerdy trim.
do you have a wide format inkjet printer? or know somebody who does? if you do, you can either buy larger-than-letter-sized pretreated (for washable prints) fabric, or buy some, so that the printed table can fill most of the center of the pillow case.