Composites have great effectiveness for creating low-volume and one-off production runs. A quick layup can be shaped over a rough foam carving before being sanded, smoothed, and
painted. Autobody filler (Bondo) is shares the same chemistry as the polyester and vinylester resins and bonds very well to surfaces that are either just hitting cure stage or are sanded with sandpaper rougher than 80 grit. The range of shapes, details, and size are almost limitless from here. This is why fiberglass is very commonly used in Hollywood movie sets, amusement parks, and sculptures.
These structures do need proper support from moving over time. Even if the structure is not meant to be weight-bearing they need to at least support themselves. There is some amount of thermal expansion that happens with temperature changes which can lead to crack development. The complexity of the geometry and thickness of the laminate are variables to consider when figuring howmuch additional support must be added. Building a frame with welded steel tubing or screwed dimensional lumber is one way to support a fiberglass structure. And another school of thought is to create supporting ribs with flat stock to hold the shape. Both of these supporting structures are commonly attached via resin and glass tabbing.