After removing the upper and lower trim pieces, I kerfed out a slot between the cabin sides and the corner post. After squirting and brushing epoxy through the kerf, I tapped in thin mahogany strips with a slight bevel for a snug fit. No more leaks, now about 5 years on.

But I have some fastening issues where the wood cabin meets the fiberglass decks. I've rebedded all of that area. I'm not sure what's going on so I may have to pull some Westie.