If you need an emergency shower knob--just print this.

Use 30% infill and enlarge to 119%

There is no "stop" on the knob (so I labeled "off") and the knob is not pretty, but it will work until you can design something better or find a replacement.

Video of operation at:
http://www.youtube.com/watch?v=_oneDFVwzkA&feature=youtu.be

Instructions available at:
http://www.instructables.com/id/Printed-Shower-Knob/