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Not exactly on topic but I did major repairs to my 1964 16' that required pouring 2 part foam. Instead of letting it expand wherever it wanted I prepared the area around the pour so I could quickly lay thin plywood pieces to capture the foam and had braces to keep the plywood in place. Also put plastic trash bags under the plywood which kept the foam from sticking.
You end up with a denser foam, and I would argue a more similar job to what was done in the factory.