Reproduce/build 4"x4" steel bases using brackets?
If you’re looking to build your own base, you could copy one of these. 

If it’s a retrofit installation, you could use Simpson RPBZ. And if it needs to stand off concrete, add CPS.

If you don’t like the looks of the metal side plates, you can use Simpson CPTZ.

For maximum resistance against rotation, you can use Simpson MPBZ.