I used a oversized piece of alum somewhere between 1/8 and 1/4 inch thick. Mounted to it, drilled a relief hole for the center post, and attached to top of the seat. I'm no small feller neither and it never budged. I've used wood in the past and though it works, I don't like the idea of water getting under it and sitting with no way to drain or evaporate. If you want to attach directly underneath the hole in the back of the seat thing works good also. Use a drywall keyhole saw to remove foam as necessary. Keep a vacuum handy. If you put the hole in the front you could mount a factory hatch cover on it or fashion a glove box of sorts there. Maybe hold a couple of the flat Plano boxes in there. Never enough storage in any alum boat I've ever been in. Think outside the box for a while before you do this and you'll come up with the best deal for you.
Wake me up early,
Be good to my dogs,
and teach my children to pray.