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tommy,

You are asking a question that is highly unusual.
How many people do you think wants to add 15' rails on a 17' foot whaler?

I have absolutely no suggestions. None...
I have never done, and never intend to do, what you are attempting to do.
There is nothing wrong with what you want to do, but it seems that you need to tackle this on your own. I don't think this is because people don't want to help, they, including myself, have no suggestions for you.

Good Luck and let us know how it works out for you.