To elaborate on what Chase and Shortyz have said, you would want to attach your skull or other item to a stock engine cover first. Then you should use some modeling clay or other product to get the blending effect you are after. Once you get pretty close to the desired end result, use that to create a resin mold from, using mold release.
Then you build your fiberglass cover into the mold you made, using gel coat as your final finish. Before final finish, you can sand and polish the fiberglass cover for finer details.