This is actually a bag I made as an added bonus for my eBay auction of an OLPC laptop. I liked the result a lot, so I've made another one for myself and thought other OLPC owners might like it.
It's basic, stylish black.
It has adjustable straps with easy-release parachute buckles - so it can be worn as a backpack, messenger bag or carried like a briefcase.
It's weather proof. I hesitate to say 'waterproof' because I haven't tested that, but it is made of a waterproof lined canvas and the closure is of a double fold design that is intended to keep the elements out.
It form-fits the laptop with just enough give that it can fit in either way up.
Possibly unique closure system. It has a side opening so that you can remove the laptop without taking it off; you just reach behind and unsnap it and pull the laptop out by it's handle. All the the closing snaps are between the bag and your body so that no one else can open the bag without you knowing.
It sits naturally in the small of your back, the idea being that the slight curve of your back leaves a space big enough for a thin bag like this. You could even, if you wanted, wear it under a jacket sort of like a laptop shoulder holster. That's actually not a bad analogy; it's possible to pull the laptop out in about 2 seconds.
The design is originally something I thought of for carrying a hardback book without bothering with a big obtrusive bag.
Step 1: First find your materials.
Anyway, here's something that is heavy and black: I believe it is called Limonto. It seems to be two layers of canvas laminated with a membrane of rubber. For 1 bag, 1 yard will be plenty.
Buckles, or something that can join reasonably heavy straps and be adjustable. I got silver parachute buckles which are way better than what I was looking for - some of those backpack strap toggles.
Snaps. You can get these from a craft store along with a tool for attaching them.