You need something to fasten the screens to. Make a flat plane with some 1x stock.you will need to scribe it into the shingles, cut a bevel on the one that attaches to the post. On the bottom you will get rot problems .
Bed (caulk) the bottom stop into the wall cap. On the bottom of the screens make sure to put a couple of screws to keep the screens off the cap. This while let it dry. If you can get away with no bottom stop that is even better.
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