A legit cruise control, like Justin's design, is probably the way to go but... Have you looked under the twist grip on your bike?
Unless it has fallen out or a PO removed it, there should be a spring tensioned, nylon-tipped, thumb screw. Look for an empty hole with threads.
Originally spec'd as a way to hold the throttle in position during timing and tune chores, the part is occasionally used by Beemer riders as a cruise control. Set it snug enough to hold the throttle but loose enough to close the throttle when needed. Think safety.
Monte