Not enough info to give a precise answer.
Remember CF is rather stiff itself, the chosen grade of epoxy also adds a bit of stiffness, but at the sake of brittleness. In a large relatively flat surface you have 4? options:
1. Increase the thickness of the cf, adds weight/cost
2. Add thickness to only selected needed areas, see #1 to lesser degree
3. Add ribs in needed locations, big increase in stiffness but requires space for proper rib location
4. Add depth to majority of the item by using an internal lightweight filler, as a sandwich, with honeycomb/divinycell/foam/mat, the last being the heaviest solution.