I second what Big said. It would be the easiest, provide some flotation IF needed, and it is the way it was originally built. I have never been that deep into my 20SF but I believe most custom and semi custom builders still use foam filled stringers today so why reinvent the wheel. AS long as it is sealed properly the foam will last plenty long. (IF you talk to Fellowship we will not be around long enough to use them anyway.
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Plus the form is already in place.