Driftwood looks sweet! If you get tired of the rock. Here is a pic of what the slate looks like on my old piece of drift wood. It is 2 plates( probbably 5X5) stacked on top of each other. If you go this route, you might need to put plate on a couple of branches.
Either way, using the rock definitely was a good idea. It will sink eventually :]

Either way, using the rock definitely was a good idea. It will sink eventually :]