When building the coaming just make sure you sand down the existing epoxy coating well-- try to get to the glassfibre without damaging it too much. This will ensure that the new epoxy you use will adhere well. I have had issues with different brands when fully cured not adhering well to each other.

Use a fine grade of sandpaper-- say 200 or above.You would think that a rough sanding would help adhesion by way of providing "teeth" but there is a possibility of air being trapped weakening the bond.

And try to increase the bond area between the new coaming and the existing shell- like a wider contact strip between coaming and shell.

And as suggested earlier , you need not use carbon fibre- standard glass will do. Maybe a little heavier but heaps cost effective.

Cheers

Prasad