re: MailBox

I've never used it,  but I would give it a try. 
I found several on Amazon,  they all say it's as strong or stronger than concrete and definitely easier to use.
Here's one

Here's one

Here's one
If you use one "post" about it.

I would probably try the Sika or the 2K,  but no experience with either,  just sharing thoughts.

Figuring out how to do something you have never done is what makes a good challenge.