usually resin is cast in a female mold that is polished and waxed prior to use. the waxing is maintaned as a release agent, and molds are often expensive and difficult to maintan
In your application, I would try to see if I could take your beak plug and impress a series of molds into a block of wax.
toilet bowl seal wax is cheap and impressable at the right temp, maybe reusable too. If it is too sticky to make a good impression, you might coat the plug with oil or some such first.
If it turns out that you need a firmer or softer molding agent the wax can be blended in a warm pan with either hard parrifin like for canning or liquid parrifin for lamp oil