easy way to do it is cover the frame with cling film then stretch some fabric over the area that you want to make the part for. You then soak the fabric with resin. When th resin sets the material goes hard and you have your shape.
Little warning. fiberglass resin stinks to high heaven. Even if you do it outside in the shed the smell will get into the house and all the surrounding houses.
I got fabby to do mine because i didnt want all the mess and smell.