Harley Davidson Throttle Lock for factory throttle control adjuster screw. 
 
This will slip over the factory throttle control adjuster screw and has small nibs to hold it in place. 
Adjust your throttle control adjuster screw until it allows free movement of the throttle. Install this to where the tab is pointing towards the rider. When you want to lock the throttle, simply push the tab forward using your thumb to lock the throttle in the desired location. You will still be able to control the throttle while it is engaged in case of emergencies. To release, simply move the tab back. 

NOTE: This is not a cruise control but rather a throttle lock. You will still need to make adjustments to your throttle to maintain your speed. Adjustment can still be made with the throttle lock engaged. This is designed to allow you rest your throttle hand on long rides and is not designed for constant use so use with caution.