Decide exactly where the top of your mantel will go. You'll probably want to place it higher than your current mantel to cover all holes left by old mantel. Measure the total height of your finished mantel; if necessary add wood blocks to the bottom of the fluted columns to add extra height. There will be one more board added to the top so account for another 1-1/2 inches or so. Cut the additional blocks to size, make them a bit larger than the fluted sides to create a small revel on the front and sides. You may want to give them a beveled edge or sand over the top to give an eased, smooth edge.
Toe nail with small screws onto fluted sides for temporary support (it will be more secure when in place). Use wood glue where necessary and fill holes with wood filler. Use a caulking gun and paintable caulk to blend in all the seams.