How did you make out Blackfly? I just made one for mine. I bought a cherry Yoke from Northwest Canoe Outfitters. The knob I used, is a round knob, from Ace Hardware and the bolt is a 3" stainless steel, Allen cap screw, fully threaded, and cut down to allow five turns into the rivnut.
The knob has the Nylock anti vibration insert so it won't come out unless a wrench is used. I counter bored the gunwale side a little deeper than
most, to in order to accommodate the clearance of the gunwale rivnut and to allow using a 1/4x20 nut held in place with locktite to keep the bolt and knob attached instead of the little washer with the O-Ring. I was considering using another Nylock nut but that would require counter boring it too deep.
The Gunwale is at a little of an angle so drilling the hole into the yoke needs to be angled or in my case, I just gave the hole some extra clearance.