Check Hellsmate Tiller Extender at Iboats if available or Amazon. There are 2 size models to choose from.
Have installed moons back the shorter model resized to be shorter for a comfy tillering needs, have added a Hardline Hour Tach and a front metal clamp to help lock it better onto the throttle grip as my comb is being powered by a much larger motor.than yours.
As any motor tends to tiller locks itself when the combo is on its way would recommend to turn the swivel bracket's friction load bolt CCW till the tiller achieves 0 load. Tller driving with an extension will be much smoother when turning tiller sideways once passing idle/fast idle speeds.
Happy Boating