As far as attaching thru bolting is always best if you have access to the underside. This allows you to use smaller diameter machine screws, nuts, and washers for a much stronger attachment. I'm not sure what you are after, are you looking for a vertical position to your trolling rods, outward/upward angle, etc. Adjustable? there are numerous inexpensive rod holders made for vertical surface mounting. It even looks like you could put a skinny flush mount if you come inboard a little. Also to consider is aft hatch interference if you want to open the aft hatches hitting the rods. So I bend a rolled aluminum pipe that clears the engine when tilted (with the jackplate all the way up) and attach rod holders to that too. Lots of pictures on the web showing those. All are custom made that I have seen.