Abstract: A trim assembly is provided for installation in a wall opening, such as a recessed window frame or walk-through doorway. The trim assembly includes a jamb extension assembly for covering the interior surface of the wall opening and one or more casing assemblies for covering the wall at one or both sides of the wall opening. Both the jamb extension assembly and casing assembly are made of solid cellular foam polyvinyl chloride (PVC) or polystyrene members having miter cut and welded corners. The specific material allows for the construction of a strong, yet lightweight pre-finished one-piece assembly for easy and efficient installation in a window or door opening independent of any window or door structure. A method for manufacturing the trim assembly and curved portions thereof is also disclosed.