I have a hot stick... mine does not have a cork ball. Mine has some type of rubber handle that comes everey once in a while. I think I have seen the cork ball available as a replacement part. This might do the job with the existing tiller extension.
PVC ... not sure if they are stiff enough. I am also concerned about the weight to stiffness ratio.
Find some type of rod build from composite material (glass or carbon) ... at the end of a the rod you could laminate a section of rod that is 1.5 to 2 inches in diameter. That should be convenient to hold. (hmmm the top part of a modern (skinny) windsurfing mast might do the job)

Patrick