Casting’s not your best option…
Hey Peppermintchild, here are some thoughts. I have experience casting.
You could make a ceramic mold, but there are other materials you can use to cast for limited runs. One material that I’ve used is Castalot (http://www.fusionheadquarters.com/product_p/castalot.htm), which will allow you to make molds that can be fired multiple times. It’s like working with plaster, but by the time you purchase a 5 pound box and pay shipping, you’re looking at $60 or more.
There are a couple of other issues, though. When casting, you’ll have to create extra room in the top of the mold to hold the glass…in most cases the glass is going to condense (usually with frit or chunks), and surface tension will give a rounded “back” (you’re casting with the bevel down) that would have to be ground off and polished. That takes time…I recommend Paul’s Coldworking Without Machines book if you go that route…you’ll have to get to a high polish on the back to match the other bevels…but still not the best option.
Another thing…when casting with powder, you don’t get the clarity that bevels will most likely need. The finer the grain of the glass, the more air is introduced, and you would end up with an translucent to almost opaque milky white color, even with clear frit powder. Even casting with clear coarse frit will produce some bubbles. Your best results would be to cast with sheets of clear glass, but even then the likelihood of getting bubbles in the casting will be very high.
My recommendation would be to pay a company that makes custom bevels to create them for you. Casting doesn’t produce the sharp edges that you’ll find on cut bevels, and having them actually cut out of float glass or other bubble-free glass will produce the best results. I am not recommending these companies—I’ve never used them—but I found Alpine Glass and Beveled Glass Arts through a Google search for “custom glass bevels.”
Just my two cents’ worth!