That's how I would tackle that, - - with 4, 5, or 6 pieces (depending on the radius and the width of the Azek), angled together at either 22 1/2, 18, or 15 degrees, (repectively). First I'd bevel the piece along it's width to get the 'basic' casing profile, then I'd cut the necessary angles 'continuously' out of the same piece of Azek, - - with the same side facing up, so as not to have any variation in shading. The edges can than be glued together with PVC glue (try to find a 'clear' primer), - - then cut the radii with a jig or band saw, - - then break out the router for some detail, - - at the very least a 'bead' on both the inside and outside edges of the radii.
Gotta' make the cuts a little on the fast side, - - after all, it's plastic.
Hope he's payin' good, - - lotta' work for one piece!!
P.S. You could probably make the job a little easier and look even better at the same time by using a 'key' up at the top.