Lynn here's a link to a decorative molding supplier
http://foamcrownmolding.com/. This by Googling -decorative polystyrene mouldings-. When I first started working we used extruded polystyrene to make massing studies/models of projects. In practice I've used it once as a curved ceiling element to reflect cove lighting.
I could see using a chair rail, crown molding or other trim piece to "frame" out a planter. Or a slices of a ceiling medallions to add a fan shaped element.
This stuff can be cut/carved using either a sharp tool or heat source like a hot wire machine or soldering iron/wood burning tool. One caution is the fumes are TOXIC (same w/Styrofoam) so read the MSDS (material safety data sheet) and work in a well ventilated area.
