I have a laptop bag for it but its a bit too bulky for most situations. Most sleeves are a bit expensive, and im pretty crafty and im going to try that guide to see how it turns out. But im going to try to make mine a little better looking.
Shouldn't be too hard. There's basically 2 aspects of a sleeve:
1) Prevent scratches. So uh, could be cloth...velvet, velour, I dunno, frankly anything that prevents random small things from scraping the case.
2) Shock/impact protection to prevent against shocks/drops. For this you need something spongy to absorb energy. So could be bubblewrap, memory foam, neoprene. Basically anything soft and spongy.
I have a MacCase for my 15" MBP and it has some good features. It opens from the side. There is a velcro piece that wraps over the end of the computer and attaches on the bottom. There are no zippers to scratch the computer. Here is the link to the website so you can see it for yourself. http://www.mac-case.com/index.html?gclid=CNOFqqCs_pACFQ66PAodPzP12w Good luck!
I'll post some pictures up when i get started. I just got back from Hobby Lobby, i picked up some thick fabric thats really soft for the inside of the sleeve and a simple plain black fabric for the outer part. Im going to try to make it this weekend.