Standard fiberglass and resin like for boat repair will typically hold up well. The key with the cotton over the mould is to layer up the cotton with multiple layers of resin to get it to hold its shape well before popping it out and layering up the inside. Use the heavy duty fiberglass matting. In the US it's called 7781 series. Orient the layers 45° to a corner. It lays in better because both length and width strands are allowed to bend. I used 2 layers of 7781 for the shroud and 1 additional layer of it at the metal bracket pickup points to reinforce it a bit. It took me about a week to make mine, because each time I scuffed and added a layer of resin, i had to let it cure. Then the ssme with the fiberglass and resin on the inside. Patience has its rewards. Keep your mould when done. You may have a need to make another for someone else. My mould is hanging on the wall of my shop, just in case its needed again.