Abstract: A sheet (10, 20) for use in a model vacuum thermoforming process, wherein the sheet (10, 20) has a thickened area (11) that may be in a preselected three-dimensional form such as arch-shape (12). The thickened area (11) or three-dimensional area (12) is placed over an area of the model (21) where the plastic sheet is to be stretched during the thermoforming process. This ensures a uniform or a preselected thickness in the stretched areas than would otherwise occur.