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I have never seen one of these decks; however if I were concerned about the strength, I would bond a piece of plywood beneath the deck.
If the area you want to reinforce is flat, a piece of 1/4 inch aluminum set into 5200 caulk would be just great.

1/2" + . Shaped to fit the bottom of the deck, sealed and set into epoxy with cloth and resin around it or sealed and set into 5200 or 4200. Actually, I would most likely use the latter method. Much less grinding and upside down work.
Make it as large as you feel necessary.

Seal all drilled holes and use backing washers.

Go fishing!