I've just had my printer a month and never made a mold or used silicone this way before. It worked great! I ended up getting Permatex Ultra Copper RTV Silicone Gasket Maker for $8 which is rated to 371°C. Based on other comments I'd read I coated the inside of the mold with some glycerin and mixed a drop into the silicone to help it cure. 24 hours later it's looking great.  Well, I think I could do a better job cleaning up some edges now that I see the picture. 😀