Forget the slate and glue. Ideally, a slate is drilled so a stainless steel screw goes through it and then into the wood. But slate is a PITA to drill and can be hard to find or to cut to size. Get yourself a glass and tile drill bit. Then use almost any ceramic tile. They come in as many sizes and shapes as you can imagine. To drill the tile you need to score the surface first. This allows the drill bit to grab and go. If you don't score it, the drill will usually just dance across the surface.
The next trick to learn is how to create space with this system that will allow one to be able to plant right up to the edge of the wood. This trick requires the use of nylon spacers. The spacer goes between the tile and the wood and the screw goes through the tile, through the spacer and into the wood. On can find spacers of various lengths and diameters. The goal is to have the tile rest on the bottom glass and then to be covered with whatever substrate one is using. The bottom of the wood should barely be buried. This creates a bed of substrate in which one can plant.
I have used all sizes of tile. For a large log I used on tile measuring 12x12x1 inch on one and and a 6 inch square to help at the other end and still had to wedge something between the frame and the top of the log for close to a year before it finally stayed down on its own.
