Here’s plans to build a mailbox. I’d just scale it larger! https://www.fourgenerationsoneroof.com/how-to-make-a-mailbox/
I would look at the US PO’s requirements for height and road set back first.
Then I would use 4x4’s with a diagonal kicker brace to hold the cantilever.
For the box itself I would look at what a used one & re-decorating it.
The post will need to be set in concrete.
I mixed my concrete in a 5 gallon bucket, set my post in the bucket, trimmed off the extra plastic of the bucket top with a saws-all and then dropped the whole bucket in my hole I dug. That way I could control the concrete curing & post position.
Do you have a preferred construction material? We have built many from brick. It's basically a rectangular box, filled up to the point of the box. Then you can either leave an opening for the mail or insert a mailbox. The continue the build up over the top.
In some places brick stands for mailboxes is not allowed. Check to see size allowed for bricked box.