We reliazed that the mould is much undersized. In order to get a near end shape, we decided to fill the big gap with gypsum, because it is cheap and easy to handle. To get it stiff and vacuum safe we laminated another ply of fibreglass with epoxy resin. Now it was time to apply spatula to reach the final shape. After several times of grinding the surface and applying spatulas we managed to fill all pores and scratches.
Then we painted the duct mould in black to get a smooth gloss finish.