Ooh, in my searching, I found a cool idea. Get a really large mailbox and a normal size one, take the door off the normal sized one and put it inside the large one, screw it to the floor of the large one. Then, pour concrete in the space around the two. Make sure you don't fill it too full, so the door of the large one stays functional. Now, you mount the whole thing and it just looks like a big mailbox.
I still haven't found the real regulation, but have found lots of interesting ideas like that.
Another one is to drive in a long metal fencepost, put aluminum vent pipe around it, and fill THAT with concrete. Mount the mailbox on it, also saw the same sort of thing but with PVC pipe, guess it depends what you had laying around, I don't think I'd like the look of the PVC pipe much, the aluminum didn't look too bad, sort of industrial. I also saw a pretty one with a LOT of bright whiteagrigate stones in it, filled a tube with stones, poured really runny cement in it, then pealed away the tube. Not sure how well it would hold up, but it looked nice.