Let me add that I tried to make one as an experiment on Wednesday. I was a one of our customers a month ago to pick of some freight and all they do is plastics of all manor. Plexiglass, lexan, etc. SI I asked them in the office what I could use that might take temps up to 350. They looked through their inventory and came up with vinyl. The specs claimed a melting point of 500 degrees. They gave me a couple of samples to try. It was a little tough to cut as I did not have a fine tooth blade for my jigsaw. I wound up using a cutting wheel on my dremel.
They suggested to try epoxy to glue it into glass as there was not much that would adhere to it. Perfect because I have some 5 minute epoxy just laying around. So I got it glued in and applied some more after it dried to seal a few weak areas. After it was dry I put it to the test. During the heating as it came up to temp the vinyl failed in a spot near the surface of melting plastic. Now this was not starting from liquid but reheating solid pieces from batches I have poured. End results were that the vinyl completely failed in one spot allowing both colors to flow together and the epoxy turned yellow and brittle. This did provide a way to get the cured epoxy off the pyrex and allow me to continue to use it for one color pours. I believe that the metal is the only alternative at this point and pyrex could make some good money if they were to alter the 2 cup glass mold to have a glass divider in it for us.