I have a M-Edge cover (one of the leather ones) for my 505, and they say that their covers for the 600 will also fit the 505. Case Logic also brings up a couple of 600 covers on their device search, or if you check "will this fit my device?".
I can't recommend the M-Edge cover highly enough, by the way. I lucked out and got mine cheap when Amazon was clearing out the 505 variety, but having held and used it, I would pay full price for it in a heartbeat if I needed another one. It feels "booky".
I bought the last one:
http://www.amazon.com/gp/product/B00...ef=oss_product
If I hadn't already bought a different one, I would also probably have picked up M-Edge's booklight. The cover has a little slot that a tab on the booklight mounts in, for easy use.
The light I have isn't quite so convenient, but it's usable as either a laptop light or a booklight (handy for my netbook) and it has a battery pack with USB output that can feed a couple of other small USB-powered devices in an emergency.
This is my booklight:
http://www.amazon.com/gp/product/B00...ef=oss_product