This is a 'simple' mantelpiece, not just because it ends at the mantelshelf and has no overmantel, but because the design is easy to execute: The pilasters and frieze are plain hollow boxes.

The beauty of the design depends entirely on the Craftsman-style entablature, which evokes the lines of a classical Greek temple. A delicate bead molding at the bottom of the frieze emphasizes the architrave position; a single crown profile, with a terminating ogee, represents the entire cornice. What could be simpler. All the moldings are from the WindsorONE Craftsman Style Collection.

The Drawings

In this drawing, the crown is removed to show the tops of the corbels.

I made the pilasters 6 1/2 in. wide, so they'd have enough heft to support the tall entablature. That means the block above the corbel must be 6 1/2 in. wide, too, so the crown can wrap around the pilaster without additional breaks.

The corbels are 11 1/2 in. tall in the drawing, which means you have to assemble them without cutting much off the width of a 1x12. I'd suggest making the corbels FIRST, then using the height of the corbels, plus the height of the crown, to establish the overall height of the entablature. That way, it's easy to align the bead molding on the bottom of the corbels with the bead molding below the frieze.

The frieze board is backed up with a 1x6 frame, which adds an additional 3/4 in. of depth.