Question: want to build an immovable mailbox

I installed a mailbox two years ago. Aluminum square tube buried in the ground with cement. Yesterday morning came home to find it ran over. My thought. I have access to a 3/12 inch diameter polished stainless steel pipe that's 8 feet long.

I was thinking about burying that piece in the ground, filing it with concrete as well as the extra around the base. And mounting the box on top of that pipe.

OR

I've searched lowes, home depot, menards for a 4 inch square hollow pipe and have been unsuccessful. I have a friend that has a welder so I could potentionally weld some smaller pieces togheter to form a 4x4 base post.

Any thoughts? Thanks