Thanks Walter, I will do the entire trunk hollow only if I am not satisfied with the partial hollow look....
i wouldnt go into any green though, you dont want to interrupt too much sap flow. i would just go as far as the wood is rotten, hold off when the heartwood appears white. you only need a dremel, with small drill attachment for this and a rotary wire brush.
maybe you can hollow deeper, but i would be mindful you dont cut off too much flow from the roots. so you need to look at the tree properly and decide which roots are feeding what areas and dont compromise them further if you see what i mean.