Would it work to add a boxed_clone() function to your Material trait that returns a Box<Material>? (or create a BoxedClone trait if you need that pattern more generally)

Implementations of boxed_clone() could still use &self.clone() to limit boilerplate.

Alternatively, if the Materials are not going to be modified and you just need multiple references to the same object, you could replace the Box<> with an Rc<> or Arc<>.