Coddy,
18/25mm Ply is adequate, it needs to be about the same size as the 'foot' that mates with the transom. Its not essential and I dont think Warrior use them, but BW had one fitted and I thought it a good idea.
The one I have taken off the new boat was bolted using self tapping carrage bolts...
Lastly, bolt heads on the outside, nuts on the inside.