Measure the width, height and depth of the electric fireplace insert. Add 2 inches to the width and depth measurements, and 4 inches to the height measurement.
Cut two sections of 1/2-inch-thick pine or hardwood to the expanded height and depth measurements with a table saw to form the sides of the enclosure, and a section to the expanded width and depth measurements to form the top. Cut a third section to the expanded height measurement, and to 1 inch longer than the width of the fireplace insert, to form the back.
Stand the back and side sections on edge to form a U shape. Check the corners with a carpenter's square to ensure that the corners form 90-degree angles. Using a drill outfitted with a screwdriver bit, drive 1-inch wood screws through the back into the edges of the sides at 2-inch intervals.
Place the top section on the enclosure so that it overlaps the sides by 1/2 inch and the rear edge is flush with the enclosure back. Drive 1-inch wood screws through the top into the top edges of the back and sides at 2-inch intervals.
Measure the interior depth and width of the enclosure. Cut a section of 1/2-inch-thick pine or hardwood to this measurement with a table saw. This section forms the support shelf for the fireplace insert.
Draw a horizontal line 3 1/2 inches from the bottom of the enclosure on the interior of each enclosure side. Install two 2-inch angle brackets, with the corner of the brackets on top, on the interior of each side with 3/8-inch wood screws. The corners of the brackets should meet the horizontal lines.
Place the support shelf on the brackets. Attach the shelf by driving 3/8-inch wood screws through the bracket holes into the underside of the shelf.
Cut a section of 1/2-inch-thick pine or hardwood 4 inches wide and equal to the interior width of the enclosure. This section forms the bottom cover. Position this section between the enclosure sides so that the top edge is flush with the top of the shelf. Attach this section by driving 1-inch wood screws through the cover into the shelf edge, and through the sides into the cover edges, at 2-inch intervals.
Determine the location of the electric fireplace insert's power cord. Using a drill equipped with a 1-inch paddle bit, drill a hole through the back of the enclosure above the shelf. This allows you to run the power cord through the back of the enclosure and to an electrical outlet.
Cover the screw heads with wood putty, using a 1/2-inch putty knife. Smooth the putty with the knife and allow it to dry overnight.
Sand the surfaces and round the edges and corners of the enclosure with a palm sander. Stain or paint the enclosure as desired.