Tim has a lovely blog (build-log!) of a wearable tricolor LED project. I liked the images of his attempt to cast them using ice cube trays. You can see the first two turned out very nicely and the remaining ones are kind of blobby. This particular effect is accidental, caused by a bubbling/melting of the casting compound. Still, it looks rather neat.
You can also, of course, make a nice mold out of milled aluminum….
I have yet to do any casting but am looking forward to trying it out