Eric from EH had this on his board building pages that he has some soft material brick, like wax, into which he just pressed the original fin vertically and pulled it off then. This way he got a hole in the brick so he had the exact copy of the original fin. Then he just filled this hole with epoxy and fiberglass stuff to get the duplicated fin.
My question is, what stuff would be suitable to be used as a mold, so it should be a soft material into which you could press the fin but naturally it should stay and not bounce back to its original form.
I haven't been able to find the wax which could be used, so other material suggestions appreciated!