Leather is an absolutely useless and even dangerous material. Hygiene in such a device is extremely difficult, and the genitals will sweat more actively. Reliability is also extremely low.
Plastic chastity belts are something else entirely. Theoretically, the cracking resistance of some polymer materials can be compared with the strength of 2-3 mm steel, which is usually used for chastity belts. The question is about the purpose of creating a belt made of plastic. In any case, the lock will contain metal elements, and if we are talking about a lock made of plastic and without a single metal element, we get a very complex object. This can be created, for example, from fiberglass or carbon fabrics, but the size will be strictly individual and the price is very high.
Nothing is impossible, a matter of expediency.
