Rear Rack for Cannondale Trail 6 with Disc brakes
Another option could be the Blackburn EX-1 rack. I've used it on my cross bike due to it not having rack mounts and it carried my bike camping supplies quite well. It also comes with the long skewer. Also compared to a lot of racks it is fairly cheap.

Might not be quite as easy to remove/install if you don't have rack mounts.

Blackburn EX-1 @ Nashbar