[3rdday]

use epoxy, it is more impervious to water absorbtion.
Put a layer or two of Biax on the inside of existing transom skin, Wet each layer of ply and fixture them together while curing. Bed & clamp that asssembly into the transom skin with woodflour and epoxy mixture , and filet with a nice radius. The glass between the sheets won't have much effect,instead put those layers on the core you just built, cut the layers about 12-16 longer on the hull sides and bottom so that you can tape that into the hull & sides that will help to unitize the hull sides, bottom and transom.