How do I Install Foam Below Deck?

I have a Mod 3 Capri 14.2 in fair shape. I am not the first owner and even when I got it about 4 years ago the bottom was a bit soft in the rear starboard side. This year it is noticeably worse and the bottom even sits on the trailer crooked as the soft spot deflect upward where it sits on the trailer bunk rails.

I assume the structure inside has deteriorated. Since the plywood supports aren't accessible the only repair I have read about is to fill the space below deck with foam.

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How do I Install Foam Below Deck?