I have the Kaoko, and I think that's about as good as it gets for a mechanical throttle lock, but in practice I rarely use it.
Throttle locks have limited utility because they are not true "cruise controls"; you still have to make constant manual adjustments for wind and grade. Ergo, I only find it useful where those factors are constant for long periods of time, which means nowhere in Washington.
I did use it quite a bit in the open West (Wyoming, Montana, the Dakotas), which are flat and have steady wind.
What I do use the Kaoko for routinely is adding a smidge of friction to the throttle grip, which makes it far more civilized on minor bumps. It's adjusted just firm enough to allow the throttle to snap back reliably, and left alone. There are much cheaper ways to get that particular feature, though.
In practice, what I actually use most of the time for speed control is a Crampbuster/Throttle Rocker device. Those things are golden. Not only reduces hand stress, but also makes it much easier to make fine throttle adjustments with this poor-resolution FI system.