We extend to you a cordial "WELCOME ABOARD !" Come on in, make yourself at home, we are a friendly group of enthusiasts, and we also appreciate the classic Chris Craft Roamer, Corsair, and Lancer boats too , as they are all on the same family tree and share much in common !
Thanks, Paul, for the further info and photo of the core plug from the bottom of your Sea Skiff, which is the same hull as the Corsair. That's an impressive hunk of material.
I understand your point about plywood strengthening the transom, both for the lifting rings (which I also have -- I replaced the weeping galvanized bolts with stainless this spring) and, presumably, to provide a rigid, flat platform for installing stern-drives. If you look closely, you can discern a flattened area in the transom where a stern-drive would fit.
On other point: You mention flush-mount transducers. I assume you have one in your Skiff? And I'm guessing that plug you photographed was from the hole bored for the transducer. Where in relation to the keel did you install it? Is the flush-mount able to point directly downward -- possibly if located directly next to the keel?