I strongly recommend drilling through the slate and bolting the piece of wood to it. Most people just use stainless steel woodscrews, I like using nylon machine screws (I drill and tap the wood so the nylon screw threads directly into it. I've also heard of people using zip ties or monofilament.
Slate can be any size you want. I think most big-box hardware stores typically have 12" and 16" square pieces. I also found a stack of 6" squares recently. I think the terra cotta tiles (or even pot trays) should work as well, they may be easier to cut/drill.
If you are careful, you may be able to break it to the right approximate shape. I recently cut a couple pieces down with a reciprocating saw and a carbide (I think?) blade. Lots of dust (don't breath it - I should have wetted the slate before/during cutting, but, well...), but pretty easy.
I don't really trust glue to hold to wood, eventually the wood directly attached to the glue will decompose. I can't remember where, but a while back I had seen a tank someone did with cork-bark tubes and sheets for terracing (siliconed directly to the bottom). It looked great, but after a few months or a year or so, the glue (or cork) gave, and they popped up. pretty much destroyed the tank. If you do go with glue, use epoxy. and maybe drill into the piece to give it more to hold on to.