You can weight it down with rocks, silicone it to a piece of slate that can be buried in your substrate, or even screw it to slate with a stainless steel screw. I have had wood that would never sink and we just siliconed it to the slate. Les, my boyfriend for those who don't know, used to make decorations for people using artificial plants, slate, and rocks. He would silicone it all to slate and all you have to do is weight the slate with your substrate!