it's fiberglass in my price range.

unfortunately i am not a materials expert so i don't know what the properties of fiberglass are. all i know is it doesn't seem to be used in any serious applications - instead kevlar, titanium or carbon fiber go there.

but it's not so much the material worries me in case of SPD as the geometry of the mount. only two screws that are so close together that you can consider them one.