A Vista Universal Throttle lock will work if you're willing to drill a small hole into what the Triumph manual calls the master cylinder mounting clamp.(The drilled hole need not extend into the handle bar.) A self tapping screw will mount the arm.

An example of what the device looks like
http://www.gunsmoke.com/motorcycling/r1200gs/cruise_control/index.html

If you need further info, I'll get some pictures together to show how it works on a Triumph grip. Very handy to have.