Well, I've always envisioned a piece approx. 5 ft. long, cut in half lengthwise (which Mr. johnrudy could do), with the flat side as the top of the mantle and the underside remaining in its natural shape. It most likely would have to be something we would need to come pick up rather than being shipped.
Sounds cool! I think an old stump would probably do, there are some cypress stumps with some of the tree left. The trick would be cutting it, it gets very hard with age.