We essentially created a scratching board for our dogs. I think we used something called step grip. (It's what you put on steps to give them a bit of grip.) It feels like sand paper but has a sticky back so you can secure it to something. We secured it to a small ramp.
When the dogs use it, it works. However, prior to making this I was strongly advised to be careful. The sanding/grinding that the dog does creates heat. Too much heat can be uncomfortable for the dog. Also, they can become numb by doing it, thus not feeling the heat, and cause more harm to themselves (sort of like quicking themselves without knowing it).
So it's a fabulous idea, but be smart about it.