Go Cruise Throttle Control: http://www.2wheelride.com/throttle_order.html Works great on mine. It is a little uncomfortable as it takes the left side of the throttle (where my hand rests) but easy to use/on/off. Fits in a jacket pocket when not used.
It's not a "Rice-Burner". It's British. It's a "Fish-n-Chips Burner"!
Did you watch the video? It does "lock" the throttle. Actually looks pretty simple and easy to use, and easy on/off. Twist the throttle to desired speed, push it up against the brake lever, and it holds the throttle in place.