Keep putting a dot of glue on top of the knob every day. In about six months, it should be about the correct length.
Nah, in all seriousness, any kind of rubber/silicon tubing would be easiest. If you wanted something a little sturdier, use tight tubing to connect a plastic rod to the knob (you could add glue for extra strength) . Look around the hardware store where you get the tubing, they should have something.