Ugh! Wrap it in a tshirt or two? Are you kidding me? You spent 2k on a powerbook and you're protecting it with tshirts?
Listen... here's what you need to do.
1. Go to an apple store/online.
2. Find this guy in an appropriate size.
3. Pay whatever they're asking for it. It's worth it.
4. Know that you have the nicest back-pack ever made.
5. Be content with your elite pro-sumer minimalism.
Sleep it, it's cool. I use a backpack when I cycle up the hill to my Uni 4 days a week. You also don't really need a sleeve, if you have a decent (read: padded) backpack.
I have the Brenthaven backpack. I love it. It doesn't seem like it holds that much untill you start stuffing it and find you can keep putting more and more in.
Outpost.com carries the Tucano Second Skin, but I found a place called Royal Business that carriyes them at a cheaper price. I ordered my second skin from Royal Business and with standard shipping I had it by the end of the week. Of course these are places in the states to buy them. I have also found many English websites sell the Skins and have heard they are very easy to find in Europe. Sorry, but I haven't searched for other places.
I was surprised by the fact that they are so hard to find in the States. I searched hard to find a cheap seller. After all of that, I discovered that the computer store at my college sells the skins for the same price as Outpost.com. Do some local store searching and maybe you will get lucky
I carry a Brenthhaven also. It is hard to beleive a couple of T-shirts are ample protection for your investment. If you don't get a sleeve make sure the pocket in whatever backback you end up getting is padded.