I made the under deck support on my Stingray by putting 3 inch strips of divinacell foam inside the hulls just under the lip. I cut them longer than the gap so they flexed up to match the deck then put a bit of tape on with resin as I put the deck on. I also added another layer of divinacell foam under the decks.
Since you are doing a special order have you considered asking them to do a double layer of foam or layer of carbon underneath the decks? The lips are firm enough, keep an eye on the weight your adding.
regards