The issue I see with a clamp-on-seat yoke is that the seat has to be in a particular position for the balance point to be correct -- and some paddlers may not like the solo seat that far forward. Plus, there would seem to be no way to adjust the yoke attachment point fore or aft as the balance point of the carried canoe may change for things you stuff inside the hull, such as paddles, ropes or dangling things. This also means the clamp-on-seat yoke may work only on one canoe and not on others. Finally, I don't see it working on sliding solo seats.
I've used the same clamp-on-gunwales portage yoke on all my solo canoes for 30 years. It has metal clamps with rubber on the clamp jaws. It slips only trivially. I can use it on all my different solos, regardless of where the fixed or sliding seats are positioned, and I can fine tune the balance point.
Finally, and this is just a personal thing, I don't like the yoke in front of me when paddling. I always attach the yoke to the gunwales behind me at an angle, and often use it as an extra thwart there to tie gear.
I'm not writing any of this as a comment on Gavia's personal choice for himself or his obvious craftsmanship, but just as general yoke information for future readers who may be deciding on what style of yoke to buy or make.