Mossback,

The solution may depend upon the type of canoe & canoe seat that you have. 

For instance, I have a Wenonah Prism.  My yoke clamps to the bars that the seat slides/adjusts on.  It was the yoke Wenonah recommended years & years ago and, frankly, I have NOT been happy with it all.  In fact, I curse it regularly on portages as it FREQUENTLY loosens when you don't want it to.  The device they used at the time to screw down & tighten did not effectively lock it into position.

HOWEVER, Wenonah has since developed another mechanism to lock down the yoke on their new Prisms.  I got to see it in action this past summer as my brother used it on his vessel.  I can report that the NEW version of the yoke locks down easily & stays locked until you release it.  He was very happy with it & could install it on his Prism in less than half the time it took me to put my inferior one on.

Bottom line: if you have a Prism OR a vessel with a seat that slides on two bars, you might want to consider their new yoke designed for that purpose.  It seemed to work well for him.  The canoe rides high on your shoulders allowing good vision of what's coming at you on the portage path.  It's a bit pricey but sometimes good quality stuff is.  Here is what I can find on the Wenonah site:   (You need to Login or Register

Again, your solution will certainly depend on your style of canoe & seat.  This solution worked well for my brother with his Wenonah Prism.

FYI

Jimbo   8-)