When you are hollowing out anything, play close attention to the color of bark, in other words, make sure not to ruin the live vein when your hollowing out a tree. Let the natural decay of the deadwood do its job to hollow itself out. The decay shouldn't affect the live vein, just make sure to monitor its decomposition. After all that how trees hollow in the wild, and it will look way better natural than tool marks. Don't get me wrong, I use power tools, but to achive a natural look, carving tools are definitely the route to go