Go to a craft store and get liquid latex. I used it to make a mold of the ports and chamber of the triple. Brush in layers then add cloth to give it body, then brush more in on the cloth. Peel it out when it dries.
You can use urethanes but need to apply a non-stick coating so you can get it out. And be careful not to get it locked in by the throat. The latex is flexible enough to work it out through the chamber.