Installing a Post Mount Mailbox on A Slab

I had a pedistal mount mailbox until recently, that's the kind that bolts to a concrete slab instead of being stuck in the ground. When it was time to replace it and my wife chose a post mount variety, I should have torn up the slab and started anew. But instead I came up with a way around this. I went to Lowes and looked in the section where they have metal connectors designed to hold different sized wood together when building a roof. I found a piece designed to take a 4x4 post and connect it to concrete. I put in an expansion anchor, tied this down, put the post on, stuck the mailbox on, and after a few hours I was done. At least that's how it should have gone. But cheap parts and the fact that a mailbox is heavy worked against me and now the thing rocks from side to side a few inches in each direction. Of course, you can't pull it out, a hurricane isn't going to take down out mailbox, but it could twist the metal to the point that it wont stand straight again. So soon I'll be redoing the mailbox by ripping up the slab and putting the stake in the ground. The moral of the story is that you are better off doing a mailbox the way it was meant to than trying to come up with your own design.