The laptop is supported on two slotted steel angles that hinge off the wall. I used some left over steel cable to suspend the frame at 90 degrees out from the wall, then when I wasn't using it I could swing the frame up against the wall for storage.
It really isn't comfortable to type with your hands zombied in the air, so I also had a wireless keyboard and mouse (I lay the keyboard in my lap and used a cheap clipboard as a mouse pad). It helps if you can touch type for this setup, otherwise you'd have to hold your knees up and look down at the keyboard in your lap.
My laptop's screen didn't open 180 degrees, so I bent the steel angles to fit it. The angles are shaped like an "L", which meant that I had to use a hacksaw to cut one end of the "L" then bend them with pliers (they're pretty strong steel, so they held their shape well). If you had a laptop where the lid opens all the way, you wouldn't have to bend the angles at all!
The four pieces of metal angle that make up the frame are held together with bolts, and I added an extra brace between the two sides of the frame to make it more rigid. I had extra metal angle, so I used that to attach the hinges to the wall, but it could just as easily be a piece of wood.
All told, it only took me a few hours to build and it worked really well!