My Turtles ignore (won't eat) the cuttle bone if the hard side is down. Reversed they swim up to it get lip lock on it and push it away, leaving a little furrow. So far they haven't eaten a noticeable portion of the backing. When the edges are stripped to the depth of their mouthes, I pull it out of the water and hit it with a dremel until the hard edges are gone. Even when it's down to smaller pieces they don't seem interested in eating the hard part.
I tried attaching the bone to a long stainless steel bolt & nut. It was great floating just below the surface. That worked for about a week but once the bone sinks, the way the tutles jerk it around, brought me to the brink of terror for my tank; so I went back to just letting the bone swim and drift on its own. When they want it they actively search for it even if it has drifted under the filter. They are really pretty creative when it comes to satisfying a want. The only thing I feel I must say: If you buy a cuttlebone packaged for birds, removing the metal clip meant to hold it to the cage seems pretty important.
I don't understand why the bones sold without the clip and packaged for turtles cost more than the equivalent parakeet bone with a clip.