A method for making a decorative polymer panel assembly for insertion into an opening of a door or window. The method can include forming a polymer grille by cutting a design into a polymer with a low-moisture content of no more than five wt %. The polymer grille can have a frame portion and a design portion. A seal can be contiguously disposed about a perimeter of the frame portion. A grille assembly can be formed by disposing a translucent panel on each side of the seal, forming an air space with the polymer grille disposed within the air space. The grille assembly can be pressed and heated to form a heat formed seal between the seal, the polymer grille, and each translucent panel. The grille assembly can then be cooled to an ambient temperature, forming the decorative polymer panel assembly.