I'd suggest very closely spaced LEDs and a diffusing translucent layer between the LED's and the clear silicon top layer. I have done this on a smaller scale with flickering firebox installations, I use four or six flickering LED's behind a plain sheet of #20 white paper to diffuse the point of light. I also sand off the front of the led's flat to further diffuse the point of light, another thing you can try.
There are some high density LED strips that have very closely spaced LED's that might work well in this application as well. Here's an example from my LED box...
If you want to get fancier, you could look into the individually addressable LED's so you could impart some motion to the stream of molten metal.
