At Copia I was looking at Swift boats and they have the same gunnel as my Rapidfire.  They sell a portage yoke they say works fine.  It looks like the standard yoke with the knob to tighten the clamp.  Where the bolt passes through the yoke it is a hole and not a slot.  So the clamp cant slip off the gunnel. They say call them they will tell you how to measure your canoe and they will make one for you.