something like one of these. When I put a spin on my 6.0, Trey gave me a part of a rudder gudgeon that you then put a pin through. It worked well. check here- something like this part 4120. This would be riveted to the main beam and the pole would fit over the part and a pin would go through the pole, through the part, then out the other side of the pole.
http://www.apsltd.com/Tree/d285000/e282886.asp