Filling a hollow part

Hello,

For a project I am making a new bonnet for a Caterham. In order to make the mold I need to make a plug from the solid model that I made. This requires filling the model that I have and then creating a shell of the inside. Ive tried many different ways of doing it including intersection and surface fills but I cant seem to get it to work.

Any help would be massively appreciated.

SolidworksParts And Features