My father made a heavy steel liner and slid it into his mailbox.It`s made of a piece of pipe for the round top and plate steel he welded together. His is 3/8" thick steel. The mail box can still be dented slightly due to space between liner and box. Replacement has not been needed since installing the liner about 15 years ago. It`s been bat swatted a few times. I don`t think any individual has struck it with a bat more than once,they learn after the first hit.