Hi there, i'm looking to form a plug for a new painted engine cover. I'm proposing to use cross section templates taken from the attached cad file, copied and cut out of 50mm celotex or kingspan insulation. I have used this method previously and once sanded to shape, covered it in car body filler. This was successful, however the skin formed over the foam plug was brittle. I'd like to be able to use a spray or paint finish that i can apply directly to the foam and flat down, or i need to skin the foam with a layer of glass fibre prior to building up coats of primer filler? If i skin the foam with glass fibre, which materials do i need to use and how much depth of construction should i allow for when cutting the foam section in order to ensure my finished plug will end up the desired size. Any advice or direction to any previous posts that cover this topic would be really useful. (The green area, as shown on the attached, I'm proposing to form out of the foam, with the pink constructed in MDF, is this a good idea or should i do it all in the foam ?). This is all for a one off item.