I was under the impression the OEM pieces are TPE/TPO like most front/rear bumper fascias are made of... it would make sense from a manufacturing standpoint because the TPE is re-grindable and can be mixed with pure pellets to recoup some of the manufacturing costs. Back when I worked in the fascia department within Nissan, we would allow a 30% regrind, 70% virgin pellet mixture.

Also keep in mind that some degree of "flex" would be desirable from an accessory standpoint, since not every customer's rear fascia is identical. You would want something that could bend to mate cleanly to any minor variation in the curve.