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Similar ebay listing of light globe I used for base.
Started off using a light globe as the base for the helmet cutting away material around the next to allow head to fit through.
A portion of the helmet was removed to make space for the mouth and the exterior of the helmet was painted to the same color as Morty's skin.
A screen door mesh was originally intended to be used as a cover for the mouth, however did not adhere well to the curved surface to an acrylic pane was used instead.