Found a nice case for the 5600 today at OfficeMax for $14.95.
It looks like neoprene (diving wetsuits) so it adds a bit of cushioning. Also has places for a calendar and business cards (or CF and SD cards if you like). Uses velcro to fasten the Zaurus if you want.
Fits just perfect. No movement of the Zaurus at all inside.
Have to take the Zaurus out of the case to use, so not perfect, but will definitely protect the 5600 in your bag and such.
I can post some pics if anyone is interested.