I found my slate at a garden shop. The slate is 3/4" thick, I have 2.5" of substrate and have the slate on the glass bottom. I found that I needed 11 pounds of weight to sink the wood while it was soaking, the slate is 19 pounds so the size was planned. I did a little research about slate in aquariums and it apparently is fine. My two screw heads do not stick out beyond the slate much but I put a piece of rubber mesh over top of the screws so they would not hit the glass, likely overkill.
50G Low Tech Planted Tank- Community Fish
120G Low Tech Planted Tank- Community Fish