I have a project 20 that I'm thinking of extending similar to what you're doing. My thinking at this point (at least one boat project away from starting on the 20) is this:
- Cut a piece of melamine board to match the outline of the transom.
- Turn the hull upside down and spray the aft 3' with PVA mold release.
- Spray the inner face of the melamine with PVA and screw it to the transom.
- Spray the aft 30" or so of the hull with tooling gelcoat and then lay up some mat and roving with some plywood stiffeners laid up in strategic locations. The layup will need to bond to the outer surface of the melamine board so it becomes the aft part of the mold.
- After cure, break it loose, flip the hull right side up, cut out the transom and prep like you've done.
- Then attach the mold so it extends behind the transom the desired amount.
- There's more detail, but you get the point.
For this to work, the steps in the hull have to be parallel all the way back. I haven't measured to confirm this, but they look to be. Not sure if this will be less work, or have better results than your approach, but it's what I was thinking of doing. More than one way to skin a cat...
Dave