There are many ways to make a negative mold. An easy way is to take a piece of MDF or plywood and cut out the shape of the object in the center. Next cut progressively smaller "layers" of the object and glue/screw them to the one cut prior. When done you can fill the inside "steps" with body filler and give it a final sanding.
Do the same for the other half and clamp them together when forming your piece.