One of my uncles kept a molded plastic guitar (called an "Islander" I think) in a closet at the beach house. Real guitars wouldn't stand the neglect and the environment. We would haul the thing down onto the sand for a campfire sing now and then, with no cares about getting sand or salt in it.
As I recall it was a hard plastic and pretty brittle feeling, probably had a high filler content. But it held up to the string tension, it was playable, and the sound was recognizable as a guitar (as opposed to, say, a cereal carton). Not great by any means but functional.
Eventually, though, it was left with the strings tensioned, the heat got to it, and it bowed up into a potato chip.
-Glenn