The joint between the metal stock and the fiberglass shell is a weak link, as they have wildly different thermal expansion. No way to avoid cracking there, and leakage into the rudder.
Personally, I've addressed the issue on my rudder by drilling drain holes in the bottom of the rudder. The water that leaks in then drains out, and doesn't cause delamination by freezing over the winter. I'm chosing to ignore the interior corrosion issue.
Helped with a 39' custom a few years back. Rudder and stock were all composite - the stock being carbon fiber, designed to lift the weight of the boat from the tip of the rudder, about 8' LOA, 26#. The rudder is corecell with epoxy/glass skin, bonded to the sides of the stock. It's held up fine so far.