We originally had a DOT-grade sign post welded into a heavy-duty truck rim, which we filled level with steel-reinforced concrete.
Unfortunately, our propane driver [an apparent new-hire] backed over it, and absconded with it to hide the evidence.
We replaced it with a 55-gal poly barrel, filled 1/3 with a gravel-water mixture, with an 8' long, 6" diameter cedar log mounted and braced across the top, with a heavy metal mailbox on the road end and a truck brake disk as
ballast on the other end.
The only thing we really need to do for maintenance is make to sure that it swivels freely on the ground and then when the snow from the town plow hits it, it just swings away freely, and we can rotate it back.
Before we realized the need to keep it broken free, it did get knocked over a couple of times, bit the loader picks it up easily, and it's nearly indestructible.
We got the heavy steel mailbox because with a 1000' driveway, the local HS kids destroyed a couple of plastic ones over the 10+ years before the steel one- because after a few years of UV exposure, the plastic became brittle.