Restrictive slider can work, but won't be stable or waterproof.
An Idea I had for something similar was to seal up the hall sensors from a Hall throttle with RTV silicon, then put them under the wrappings on the handlebar. Then put the magnet from the throttle into the finger or palm of a riding glove. The throttle would work when you moved your hand between those two points. ( Slide, twist, how ever you wanted to position the sensors). Also, the bike would be unridable with out the glove, adding some security to it.