if you made ribs underneath it running longitudinally.
hollow CF tubes bonded on would work.
balsa rods covered in CF
birch dowel covered in CF
1/4 in or 6 mm dowels or tubes should provide more than enough.
you also could try a rib on either side along the bottom of the deck.
if you started as a base to explore with, make the ribs the same
thickness as the deck. they could be solid or hollow.
you could also try 4 to 6 ribs fashioned from dowels cut in half lengthwise so
the final look is similar to a cardboard corrugation or sheetmetal corrugated roofing.
example here is the SR-71 Aircraft has a section like this on the upper rear of the fuselage.
i assume partly for stiffness and partly for increased surface area to radiate heat from the
friction from the air as it flies at high speed.
i am not familiar with the height and width of ribs and which shapes will give the best performance.
that would be lot of complicated math that i am not so good at.
http://www.cstsales.com/carbon_te.html#halfround
CarbonFiberCreations