The best way to stiffen your casting deck would be to add a layer of marine ply. Yes, carbon is stiff, but in that particular application it would be much less effective than ply (and probably more expensive).
Actually a microskiff member turned me on to a gent who Fabs the assembly completed with hinges and guttered frame. I think he charges +/- $300.00 each, a few selections/sizes. Still think I will pre laminate, 1208 or 1708 before going under the saw for the hatch openings. Rather be safe than sorry. :-/