Between this and the cardboard rough shape it should not be too tough to make 2 layers of transom to fill the space vacated by this mess.

It turns out the transom was not full of rot, just a small area to starboard was rotting, or starting to. It was no wonder it didn't show much flex when pressure was applied. It was delaminated and a bit warped and definitely had a very high potential to come apart soon, and needed to be repaired, but the wood was not actually decaying for the most part. Ugly, messy, becoming structurally unsound - but not soft and rotten.

This is the view as though you were looking forward from the back. The right side of the picture is the starboard side of the transom - you know, that area where the water would pool from the boat sitting cockeyed on the trailer. You see where it was sitting soaking up water to the bottom right of the photo.