The extension bracket needs a tweak . . .
After much effort, the extension bracket would not come all the way flush with the boat's transom. I suspect that the bevel sides are a 'wee bit' thicker/wider than my original take-off from the transom, even though I made some accommodation for fiberglass thickness. It probably was not enough.
Anticipating such an occurrence, I figured that I could add a spacer, which would then let the extension bracket mount in a slightly wider space. It looks like I need about 5/8" (15-16mm). I have 1/2" and 1" Coosa board to work with. I'll see what adding a spacer does. (tomorrow's effort)