Have you considered just getting a Pelican (or similar) case that has the foam inside? You could cut that to fit, or you could get the Pick-n-Pluck style and just pluck out the shape you need. That'll definitely cover you for protection, and they're relatively lightweight, being as they're designed for traveling already. The 1450 or iM2200 would be the right size to give you good padding on all sides, and enough room that you could cut a second slot into the foam to hold cables.
EDIT: one other nice thing about these cases is that they have dual locking holes, so you can secure your keyboard even further. ;)
Pelican 1450 (http://www.pelicancases.com/1450-p/1450.htm)
Show Image
(http://www.pelicancases.com/v/vspfiles/photos/1450-2.jpg)
Pelican Storm iM2200 (http://www.pelicancases.com/IM2200-p/im2200.htm)
Show Image
(http://www.pelicancases.com/v/vspfiles/photos/IM2200-2.jpg)
Both can be had for around $100 if you shop around.
Thanks for the idea. I looked at Pelican, and I wasn't sure about the size. I will definitely give it a second look. I picked up some soft cases from amazon that I will post shortly.