After mocking up the fireplace and chimney breast in foam core board, I started building the final pieces using wood and egg carton bricks.

I cut the opening in the fireplace front and lined it with thin strips of wood.  I'll cut new egg carton bricks that are long enough to wrap around the edge made by these strips to give the illusion of solid bricks lining the opening.  The sides and top of the fireplace structure were then glued in place.

I then built the firebox to fit behind the opening.

I will likely line this with egg carton bricks as well but in a different color than the outer brick.

I then measured the opening needed in the chimney breast to fit the firebox.  After cutting that opening, I attached the sides.

The chimney breast will be permanently affixed to the wall and wallpapered, but the fireplace, firebox and lighting unit used to simulate flames will be removable.

Since I'll need to paint and grout the bricks on the front surface a different color from the trim, I taped the trim pieces to cardboard and painted them separately.  Touch-ups are easier than trying to mask off entire areas.

Here the egg carton bricks have been glued in place and are ready to be painted.

And, after painting, grouting and attaching the final trim, I have a finished fireplace mantel in the exact fashion I wanted.

click image to enlarge