Chain saw.

Just kidding.

The suggestions above are all good ones. The tree does sound far enough away that you shouldn't have too much trouble turning inside it. It's hard to explain how the brain does depth preception beyond the point where the seperation of the eyes doesn't help any more (not far, though I forget the number). Anyway, I can tell you for sure that depth perception with the models is a learned skill, and yours will get better with practice.