Abstract: A three-dimensional fireplace insert which is intended to be placed in the firebox of a fireplace that is not in use and to substantially seal the opening to the fireplace. Such insert may be decorated to enhance the appearance characteristics by painting or printing decorative material on the exposed inside surfaces thereof and/or by placing a decorative object or objects therein. Various members of the fireplace insert may be formed in one piece from a suitably shaped blank of a suitable material by the folding of such blank, and a bi-axially extensible frame may be provided to be positioned within the opening of such an insert to facilitate the accurate positioning of the front edges of the insert with respect to the fireplace opening. Organic materials, such as thermoplastic materials and fiberglass reinforced thermoplastic materials may be used to advantage in the manufacture of the fireplace insert.