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Another option to try is a 2:1 reduction by adding a pulley to the gearbox lever, the cable goes around the pulley and fixes back to the gearbox. I have seen one on a beamer but you could make something similar with a short length of bicycle chain and sprocket so the cable gets a straight pull.
I have seen those devices advertised but if it is a 2:1 reduction in effort wouldn't it require twice as long movement of the lever? I find that I must have my lever adjusted with minimum free play and then pull it all the way to the hand grip. Otherwise shifting becomes difficult. I don't see how I would be able to use a 2:1 reduction even if I could fit it on.