Hi Jon
Do you recall how they made barns about 150 years ago, with one or two dowel pin(s) holding all the timber locked in place...they are neat looking and strong...
Without the need of any metal fastners..
You may ask how to do it...this is what I would do
Do the 2 x 6 on the router table that way you will get a nice clean cut on the 2 x 6 then get the saws all out to cut the 4 x 4 or to say put the slot in the 4 x 4 post..then once it fits just right clamp it up and dril the 2 holes out with 1" spade bits then drive one pin in then the other unclamp it and your done..
But YOU still need to dig the holes for the 4 x 4 post and just pack some gravel around the 4 x 4 so the water can run away from them other than soak in to the 4 x 4's
Have fun..
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