Howdy,
There are a few things to consider:
1) Is it really driftwood, or is it a piece of dead wood that never saw water before? To be suitable for aquarium use, wood should have spent a significant period of time (years) in a flowing body of water or in a swamp. Anything else will have a hard time getting waterlogged and it will also rot, greatly affecting your water quality.
2) conifers are not the best wood for a tank, since they leach more compounds than deciduous trees.
3) if you go for the slate option then make sure you use stainless steel screws.
4) I tied mine down with fishing line, weighing it down with rocks tied to its bottom. Do not pile rocks on your wood, since it might pop up to float and the rocks fall against the sides of your tank. Depending on the depth of your tank and the buoyancy of the wood, it might even hit your canopy
Good luck,
HarleyK