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to the OP large piece (refrigerator box) of cardboard is your friend. You could make a pattern at say 3 points of the bottom hull.
Transom, i/3 from the front and one in between these two. That will give you the points of where to support on the inner chines.
Of course you have to be able to lift the boat and get under it. Maybe a come-a-long in the boathouse would help you with this.
From what I know the boat is mainly supported by the 2 bunks and not the keelw (except maybe a roller or two.