Is a low-temperature metal (similar to lead in melting temperature) that can be cast in RTV rubber molds. If you can find some old linotype slugs or damaged type from a letterpress printer, you might try that. Type metal has very little shrinkage.
In my experience, however, there is a high failure rate with most home casting methods, including type metal, resin and those little meltable plastic beads that they use for fake stained glass projects. If you can buy the part you need, it's usually much cheaper and easier.