This is usually a pretty hot topic.
IF your mail carrier is that bad, call your local postmaster and file a claim. Thats first and foremost.
Second, gotta have a box thats "postmaster" approved. Meaning if you make some homemade conglomerate they can either make you remove or refuse to deliver your mail. OR worse is liability if you have a 50# mass of steel on top of a pole, and someone swerves and takes it out (animal in the road, slippery road, kids, etc) and that mass detaches and goes through a window and kills someone.
And third, the post. Same liability as above. As fun as it sounds to stick a 12" diameter solid steep pipe 10' in the ground and surrounded by 5 yards of concrete......your @$$ is in hot water if someone gets in an accident and injured as a result.
Your mailbox (typically) is in the state, county (whatever jurisdiction) right away. Gotta abide by their rules.
Go to lowes. They have heavy duty mailboxes with 1/8" steel instead of the thin sheetmetal. Get a mailbox post from there as well. If something happens.....you are covered from a liability standpoint. And those heavier duty mailboxes, they will survive just fine. Might have to replace the post on occasion....but better than killing someone for getting into an accident.