Here you will find all the rear bumpers you could possibly get for your car!

The original panels were injection molded plastic. Stock bumper looks like this.

Fiberglass rear bumper
It can also have a diffuser attachment. Can get new from AUTOGIANO.
https://autogiano.com/list/ma/list_mreb.html

Part of the RICE ROCKET R body kit. Has two vent holes not sure if you have to modify the bumper reinforcement to improve the airflow.

Side Sills

Rear Panel