Hi Craig
The backing plate is normally built into the deck where the pad eye would be fitted for the spinnaker downhaul. If you have no pad eye fitted you will see a flat area on the deck just in front of the forward hatch, directly below this within the deck is a 6 or 8mm thick aluminium plate that can be drilled and tapped to take the fixing. Not sure if they are designed to take a forestay load though but a spinnaker downhaul loads up significantly so you should be alright.
Regards
Rick
