Fireplaces are usually the focal point of a room, so you probably don't want to cover a fireplace with just a plain piece of plywood. To cover the fireplace permanently, brick in the opening or frame your own movable cover with wood and set in place. Alternatively, construct a temporary cover only for the times your fireplace is not in use. Along with closing the flue when the fireplace is not in use, a cover can help keep the cold air out of your home, decreasing heating costs and wasted energy.
If you use the fireplace daily and want an overnight cover for when the fire has gone out, make one out of a nonflammable material. There may still be hot embers in a recently extinguished fire, and you should not put anything flammable near smouldering embers. A suitable material for a temporary cover is tin. Use a thin sheet of tin and cut it to size with tin snips. While it is not the most appealing looking cover, it is safe. Make the cover more attractive by embossing a design across the cover.
Use a piece of plywood to make a cover that will sit in front of an unused fireplace. Cut the plywood to size and cover it in a fabric that coordinates with your decor, or paint it with a freehand picture or stencils.
A more elaborate design idea is to use some old ceramic dishes or pottery, break them into small pieces and use the pieces to create a mosaic design. This will make the cover heavier, but it will look attractive once it's in place. If you don't have any suitable ceramics, look for inexpensive plates and vases at second-hand stores.
Styrofoam sheets that are sold as insulation can also be used to make a lightweight fireplace cover. This is an easy method to use when creating a fireplace cover since the styrofoam can be cut easily with a knife. For extra insulation, glue a piece of foam rubber, the foam used to make furniture cushions, to one side of the styrofoam. Or double up the styrofoam to make your cover thicker. Cover the sheet with a decorative piece of fabric. This type of cover will go up in flames quickly, so use it only to block an unused fireplace.