Here is a hillybilly mail box about 50 feet from the mail boxes where mine is. All it needs is some tie wire, like what we fix tie rod ends with around here, wrapped around it to complete fastening it to the tree.

That was my mail box for a while. But one day, the postal inspector said it won't work because there was too much of a risk of the rual mail carrier running her car into the tree delivering mail to the box. I've recently put one up on a steel post.