I had my dark cloth made from ripstop nylon...black on one side, white on the other for the reflection of sunlight in the summer. I had two pairs of velcro strips about 6 inches long the width of my leather camera handle sewn to the center of one of the sides. When setting up, it is a simple matter of passing two pieces of velcro through the handle and fastening to it's opposite componant. This has worked well for over fifteen years now.
Another idea that I have seen and like a lot is to fashion one side of the darkcloth into a sewn together circle with elastic on the interior of the circle. The circle would need to be large enough to slip around the perimeter of the camera back when it is stretched and would then grip the camera when relaxed. This seems to be a good idea in that it would do a better job of securing the sides of the darkcloth then my arrangement does.