I've been using an older version of this case for several years, and I've been very pleased with it. (It was designed by an airline pilot.)

The laptop section splits off from the main section with a zipper, so you do not have to take your laptop out of the case when you run it through the x-ray machine.

They come in sizes to accommodate 15.4" or 17" laptops.

ProStyle? III-XF Shoulder Case | Brenthaven.com