Carefully drill and Tap a 1/4-20 hole in your throttle clamp right beside the grip. Get a nylon 1/4/-20 machine screw and screw it in to hole the throttle open. Do not use aluminum or steel machine screws. The cable is behind there and you will damage it with metal screws, be careful when drilling as well.
If you need a little extra friction to hold the screw in place when it is loosened from the throttle, wrap with Teflon tape then screw it in.