I have fitted a helm impeder as described on the old DCA web site http://www.dca.uk.com/articles/himpeder.htm
The design is simple and works well but I discovered a problem with the way I had implemented it.

I have included four photographs which show my modified version with the last shot showing the problem.

The first picture is a plan view showing the control lines (red) led to the aft cleats.

Picture two is a side view showing the major components.  The elastic adds friction to the system and is tensioned by pulling on the control line which is then cleated in the cam jammer cleat.

The modification is simply the addition of the fairlead by the cleat and a stop knot in the correct place.  The key thing is to ensure that when all the tension is released that the hook does NOT move forward and hit the forward fairlead.  If it does, as in the last picture, the helm will jam - often with exciting results.