How about taking out the guts of a 17" Dell XPS laptop and connecting its screen to the insides of a Sony Vaio ultrabook or a Macbook Air? That would give you over a quart of volume in which to pack contraband.
Not necessarily. Since the Macbook Air doesn't have a hard drive, you could disguise some kinds of contraband as the batteries, hard drive, and CD drive. Newer X-Ray machines might have the configurations of known laptops known by the software, but I doubt the personnel would be able to tell the difference if some effort is made to disguise the x-ray images.
"might have the configurations of known laptops known by the software"
Given the sheer quantity of hardware thats ever existed and the workflow I don't think this is a serious possibility.
I know pretty much every piece of ham radio gear every constructed has at one point gone thru the xray for "dxpedition" people. Along with pretty much all consumer electronics. It would be a heck of a lot simpler to mod a COTS radio from Radio Shack.
I hope this comment didn't put me on some watch list.