We recently purchased a Macbook Pro and needed a sleeve to help protect it from scuffs and dings. I finally had a little time on my hands so I spent less than an hour to whip out two Star Wars sleeves.
I picked out some flanel material and decided to put some scrap batting in between to give it just a little extra protection. The material was wide enough that folded over it would work for both the inside and outside pieces. I cut 17 inches long, but in the end could have easily cut it about 15 inches.
Sew along both ends leaving about 1/4 to 1/2 inch seam and trim each end of any excess fabric. Flip the piece right side out and iron along both seams, to give it a nice finished look sew along both seams.
Take the MacBook Pro and fold over. Make sure the side you want facing out is on the inside. Use pins or clips to show where to sew along the two sides. For me, this ended up being about 14 3/4 inches.
After sewing along each side, turn it right side out and see how it fits. At first I made a few adjustments to both before I got the right fit.