I'd probably use that Clear epoxy stuff that is often used for coating tabletops. You can get it online or at home depot usually.
As for a mold and that thread adapter....hmm.
For the mold, maybe a pingpong ball, something you can easily cut off once the casting is hard.For the thread, you could thread it onto a screw, seal the end somehow so that the epoxy wouldn't get into the threads, and then suspend it into the epoxy once it's poured? It'd be tricky, but it's a cool idea, though.
Maybe a craft store like Micheal's would have some equipment for casting like that. It was a fairly popular thing to do a while back (put scorpions, or spiders etc into the epoxy as paperweights...)