I cheated and bought a half yard of pre-quilted fabric at Jo-Ann fabric for $ 4.00. Then I basically sewed it into a pillowcase, which took almost no time at all. Also, I should preface this with: I don't really know how to sew.
Pin the open side of the pillowcase and sew across the "hem."
Fold in half to make the pocket. Sew the remaining two sides. Remember: the pillowcase is currently inside out, so make sure you sew it as shown!
Flip it inside out, and voilà! A complete laptop cozy. I made a smaller one for my Kindle, too.