This is a slight redesign of the Motorcycle throttle lock by toofuu (thicker, more rigid, with glove friendly chamfers).
I added a thumbscrew that works with an M4 bolt/nut to tighten the throttle lock if its grip starts loosening. The thumbscrew is mounted with some interference to keep it from slipping.
Use a stainless steel M4 bolt (20mm) and a standard nut.
If you are looking for a handguard compatible version, find it here: http://www.thingiverse.com/thing:1329289\
IMPORTANT SAFETY WARNING: make sure that the head of the bolt fits in the hexagonal recess in the bottom rail of the hinge, and is flush with the surface where the brake lever slides. It is dangerous to have any protrusion that might interfere with the brake lever operation.
Before you try the throttle lock on the road, familiarize yourself with its operation, adjust the screw tension, make sure nothing interferes with the normal operation of your controls, etc. you don't want to find out something is not working right during a panic brake....