Anyhoo, the project I'm working on is a small, removeable mantle for our fireplace. I'm using an aluminum bracket to mount it to the wall (french cleat). It's 5' wide, and an L-shape that's 8"x8". I'm going to use a box joint to glue the two pieces at right angles (think interlocking fingers) all the way down. There will be a bracket on each end to lend strength and support. The panel against the wall will be a panel in the true Arts & Crafts style. Here's my drawing:

To give you an idea of what it's going to look like, here's my buddy Schroeder's from LumberJocks.com mantle. Mine will be sort of an upside down version of it.