I would NOT recommend the Helsmate tiller extensions from Iron Wood Pacific suggested above. When I got my duckboat I purchased one of these and they have a serious flaw. While the shaft is aluminum, the part that goes over your outboard tiller is made out of plastic and only attached to the extension shaft with one screw. When turning using the Helsmate, all of the stress is put squarely on the plastic where your outboard tiller ends. Mine ended up bending while running the boat and almost caused me lose control.
I went out and purchased a section of white PVC pipe and cut it to the length I desired. 6 inches from the end I drilled a hole through the pipe, then cut the pipe in half to that hole (it kept it from splitting more). That end slides on my existing tiller shaft and I used two pipe clamps to hold it on...... works great. I finished it all off by painting the PVC.