Gonna try a picture....if it doesn't work, I'll describe what you missed.
The Mrs. Said it was time for a new mailbox and asked if I'd but up a new one. She was thinking the standard wood 4x4 but I ran to the shop before she could give directions.
Started with a purchased locking mailbox and stripped the paint off the back, top and sides with a stripping wheel. Then fabbed up a frame with some square tube and then added the rebar. Removed the mailbox and shipped off to sandblast and then had only the square tube powder coated.
The frame was then mounted on a cement slab and filled with 600 lb of rip-rap. Reattached the mailbox then chemical-rusted the rebar and mailbox sides/top/back with hydrogen peroxide and vinegar.
Turned out pretty nice....the best compliment is when the door bell rings and someone wants to know where I "bought" it.