[Destroyer]

IF you really want to use a full backing plate, and IF there is sufficient room under where you want to mount your seat, there is a way to do it without having to reach under the deck. It sounds complicated, but it's really very simple. Just involves a little work

1) Place your pedestal over the metal you want to use as a backing place. The backing plate metal MUST be at least the same thickness as whatever size bolts you intend to use to secure the seat pedestal. Scribe a circle on the backing plate that matches the outside diameter of the base of the pedestal.
2) Place the pedestal on the deck exactly where you want it to go and using a pencil trace around the base of the pedestal mount onto the deck and also trace the bolt holes through the mount onto the deck.
3) Use a transfer punch through the holes of the pedestal to exactly mark the place where your threaded bolt holes will be in the backing plate.
4) Drill the proper size holes in the holes in the pedestal and thread them. (Probably 5/16 or 3/8, depending on what size bolts you intend to use). Cut the backing plate into a circle shape Just slightly smaller than the scribed line that you made in step 1.
5) Using the previously marked bolt holes on the deck as a guide, place the backing plate exactly where you want the seat to go and mark the deck through the new bolt holes of the backing plate. Drill through holes in the deck one size larger than the bolts that you intend to use. (5/16 bolts get a 3/8 hole, etc)
6) Cut a slit in your floor between the holes just long enough and wide enough for the backing plate to fit through. This slit will be completely covered by the pedestal mount base once it's installed.
7) Drill a small hole in the center of the backing plate.
8) Thread a strong cord (Paracord works great here) through the center hole in the plate and tie it around a washer bigger than the hole you drilled in the plate center. On the other end of the cord tie a large nut wider than the slit in the floor. (This is to prevent the cord from accidentally falling completely through the slit).
9) Squeeze a liberal amount of construction adhesive onto the side of the plate opposite the washer side, then slide the washer and then the plate through the slit in the floor. Use the cord to pull the plate up flush with the bottom of the deck, while using a pencil eraser to position the backing plate holes in line with the holes you drilled into the floor.
10) Place a bolt thru each of the deck holes and thread them into the backing plate holes. Once you have all the bolts threaded into place tighten them all to pull the backing plate firmly up against the underside of the deck, thereby gluing the backing plate to the underside of the deck in exactly the correct position.
11) After 24 hours or however long it says for the adhesive to cure return to the job, remove the bolts and make sure your plate is securely glued into position. If you're satisfied that it is then cut the cord and let the washer fall into the void beneath it, squeeze 5200 into the slit to seal it, then position your pedestal seat mount over the bolt holes and bolt it into the backing plate.
12) YOU'RE DONE. GO HAVE A BEER OR TWO.