Made out of 3/16" thick HDPE plastic. Used a heat gun to heat the plastic and form into shape.

Advantages over a store bought throttle lock:
Cost less, used a piece of scrap plastic, cost = $0.
Adjustable clamping force by changing rubber band.
Offset arm provides more room for hand on brake.
Easier to set with index finger.
Ratchet action makes it easy to set and release but hard to engage unintentionally or by gravity.