I knew a guy that kept having his mailbox hit so he made a big one up with 4 or 5 slots in it for all his neighbors out of 1/4" steel. Set it on a pipe which sat in about 2 wheelbarrows of cement. That lasted until someone wanted to test out their new winch late one night. So the next time he borrowed a back hoe from someone he knew and dug as deep as he could then called the cement truck up. When he was done he proudly stated that it would take a D8 cat to pull it out now. 25 years later it's still there.

I know of someone else recently who was having the same problem. He went and bought the biggest box you can buy. Took it to work and had it lined with a rolled piece of 1/4" steel. They're up late one night watching TV and hear a "DONG" go our the next morning and there's a really small ding in the outer box and a couple pieces of aluminum baseball bat in the road.
No one else has ever tried to get it, although somebody had a wide canopy or camper and clipped it once. Ripped the door off the front and left fiberglass all over the road.