Here is a URL for a lightweight hard case for the PSR-s900:
http://www.midi-store.com/Guardian-Featherweight-keyboard-case-p-16494.html

It won't fit in the trunk of my car so I am sticking with the Kaces 9kb:
http://www.kraftmusic.com/catalog/accessories/index.asp?product=1219

If you use the kaces, make sure you put a piece of foam on top of your keyboard so that you don't damage your LCD. I did this once with my PSR740.

I also use this stretchable Gator cover to avoid scratches and so that pieces of foam don't get into the keyboard:
http://www.amazon.com/GKC1540-61-Key-Keyboard-Cover/dp/B0002CZTIE

Beakybird