There may be an off the shelf product that will work. Simpson does have some post foot connectors. I can't really sell you what you need -- allthread, nuts, washers, etc are pretty easy to come by. But really, I don't know what you need for this specific project.

I feel like I should explain why I am hesitant to answer your question with specifics. I do this for a living and I take structural design quite seriously. There is a lot of risk in building or designing something. That's why I have insurance. And that's why on almost all of my jobs I pay an engineer to answer these types of questions for me. In other words, I don't even answer this question myself when I am getting paid to -- I hire a legally recognized expert (licensed engineer).

I don't mean to offend or seem unhelpful -- I just wanted to explain where I am coming from.