Kuryakyn Mechanical Throttle Bar End Cruise Assist
This may be the simplest, cleanest and most effective throttle position holder on the market. It attaches to the end of the handlebar like a bar-end weight. You just open the throttle to the desired position, then roll the throttle back and a spring-loaded tensioner keeps the throttle in place. Then you simply roll the throttle forward to disengage. The matching clutch side Bar End Weight is also available to complete the look. 
Features:

Chrome finish

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Note: Installation may require cutting or drilling a 5/8″ hole in the end of grips.