A few weeks in advance of a long trip to Memphis for AV, I had been looking at a variety of options for allowing me to let my throttle hand relax on occassion during long segments of riding. There are a few options out there, but I settled on the Go Cruise throttle lock. There is a plastic and a metal version. After looking them both over carefully online, I opted for the metal version because of it's adjustability and durability.
As you can see from the pic, it a rather simple device that works by holding the throttle open via friction when the narrow end is spun against the brake lever. To install, simply take the silicone band and wrap it around the handgrip on the throttle side. Then clamp the jaws of the Go Cruise around the band and adjust the clamping force with the adjustment wheel until it just barely holds the throttle open when the arm is spun down against the brake lever with your index finger. By adjusting the wheel with just enough clamping force, it ensures you can easily close the throttle with a light twist of your wrist.
My experience is that it worked perfectly over the course of the last 2000 miles over the last few months. It allows me to lock the throttle open at my desired speed and then relax/stretch my hand and arm.