Thanks for all the compliments there guys!
The outside of the stringer and the kicks are covered in Azek rim cover. I lay the rim cover on and flush trim router it to the stringer, makes it super fast and a tight fit.
As to attachment, the board are riddled with face screws and Cortex plugs. We make sure to line up the grains so that they disappear.
As for expansion and contraction, this stuff does move! I measured 1/4" of movement and I'm sure it could go more. Depending on the climate installation would have to vary.
In my case I control the joint with face screws, screws through the miter, and glue in the joint. I also let the board have a bit more slack away from the miter to allow it some place to go if it does decide it's moving.