Mike, that is basically what I am suggesting, with a similar method to connect the post to a sill on the floor.
T, thanks for the advice, after doing some research it does seem easier to me than I thought. I have found many examples of plans out there. However, this octagonal structure seems hard, and I have found no plans that could help me make my own. Do you know of a place where I might be able to find something like that? Even a place with info on the joints I would need. I was playing around in Sketchup and came up with the following (attached), but I don't know if this is a good joint or not, for example.