In the undone state, sitting on a piece of cardboard, I decided just how I would preserve it. I would take the 'unfinished' puzzle and put it in a see through frame. But first we had to figure out how to get it home as intact as possible.
Now home, I set it up on my dining room table. I surrounded it with some edge pieces and was ready to insert it into the waiting glass frame.
Phew. Done and hung.