Azek trim on bay window box
Just to follow this up - I ended up using 3/8 ply for my backer, then a sheet of azek 3/8, then 1x6 on the left and right vertical, 1x8 o nthe top and bottom which makes a nice box effect.  Then trimmed the inner box with azek scotia.

I'm not accepting this answer, since it is mine.  But everything worked out just fine.