It certainly looks like a winner. Carving a flap out of a single piece of solid wood will probably get you the worst possible stiffness/weight ratio. The carbon fiber should stiffen things up dramatically -- I assume you use mat with random fiber directions, but if you use cloth you should run the fibers diagonally to line of flight (45 degrees if it's woven at right angles).
It'd be interesting to see someone build flaps in several different ways, then measure torsional stiffness and weight. Covering should not be ignored! I can see several different ways that could be investigated:
1. Plain ol' wood. This is probably the worst.
2. Plain ol' wood with silk on a 45 degree bias. Should strengthen up, but I dunno how much.
3. Plain ol' wood, silk on 'straight' -- if silk on a 45 degree bias makes a difference from plain wood, this will be better than plain wood, worse than 45 degree.
4. Plain ol' wood, silkspan.
(if you're thorough, you'll try different weights and or multiple layers)
5. Plain ol' wood, carbon fiber. Anything with significant CF should be way stiff, but fragile if you really twist it, particularly if you stick the CF on with dope.
6. "balsa plywood" -- stick two sheets together with grain offset by 30 degrees or so, so that top and bottom grain both run mostly along the flap, but at a 15 - 30 degree angle. Then sand it out like plain ol' wood, with the glue joint down the center. I would expect this to be significantly stiffer than plain ol' wood.
7. Built up -- leading edge like yours, ribs, sheeting.
7a. Built up, diagonal -- Run the ribs diagonally to make triangle pattern, sheet.
7b. Built up, diagonal, no sheeting. Done right I bet this'll be stiffer than plain ol', and it'll be lighter with careful glue management.
7c. Built up, with CF -- this is, I think, what you're proposing.
8. Foam core. Like yours, only with foam instead of ribs. Probably more trouble cutting the core than it's worth.
8a. Foam core with/without CF.
As a control you could mill a flap out of tungsten. It'd be stiff, but I think the weight would disappoint.
If building and testing 12 different flaps weren't enough, there's going to be a distinct tradeoff between flap thickness and torsional stiffness. Who wants to make 24, 36, 48 flaps?