Cottonsail has it right. If you're gonna do this at all, the Cottonsail method avoids the need for structural modifications, and it adds area to the control surface. If you tag on a "tip section" like this, the internal structure of the existing rudder will be quite up to handling the addition area.
A consequential problem is that lines from people's crab pots and other such can not slide harmlessly off the bottom of the rudder when you snag them - as you will :-) - but will stick in the gap twixt skeg and tip section. That would be a problem for me. I don't enter water
unless I can see the steam rising from it ;-)!
As for a NACA ( National Advisory Committee on Aviation, ca. 1928) "foil", there are dozens of them, and none of them are intended to "back up". Aeroplanes don't back up while they are in the air. The hydrodynamics of the flow around the rudder blade will shift so drastically when making stern-way that considering any particular "foil" a waste of time given how little time we spend going astern.