[Mr. Sandman]

If you are going to be walking up that narrow gunnel offshore...you need a railing or at least a set of solid handrails. The "naked" look may look clean but you can not walk up the side of a boat like that when underway offshore without something. I would consider embedding plates of G10 that are epoxied in that slot. This way you could bolt or screw into something really solid and you could make the plates as large as you need to make sure you have plenty of surface area to insure it is secure.

That looks like a nasty job and you are doing the right things. I am surprised you can't get underneath that from the inside but whatever you do make it as solid as possible and west-system the hell out of it. Avoid wood... well now that I think about it by the time the wood rots again the boat will be near the end of its life so I suppose you could use wood for ease of installation .
Best of luck with that job!!