How Do I Build A Floating Shelf?

We have an electric wood stove, in our family room. I have ALWAYS wanted a mantle, so that I could decorate and hang things above it. The only problem we have, is that it is in a corner of the room, with a stove pipe up through the ceiling. I was wondering, do you think it would look odd if we hung a "mantle" or floating shelf over our couch, which is on the center wall? If not, how do I build one?