The silicone molding materials I use are from Polytek are only good to about 230 C. I have not used Weicon 300 but it is supposed to be good for temperatures of 300C. Weicon HT 300 red silicone is highly flexible, with an elongation at break of 500% so might work for a single piece mold.so the silicon has to be able to sustain 200--300 C.
Allan