Just done mine, used P40 but you have to make sure the fibreglass is spotless so it bonds to it properly, I used malleable alluminium grille so you can easily mould it to the necc shape, hold in place with tape, apply P40, remove tape apply more P40, paint satin black.
Whilst this seems like a good idea, won't all the debris just collect between the bonnet and the grill on either side? (see the little bit of leaf in your picture) Making it harder to clean?