When shopping for my wedding band, I saw titanium rings that looked like that. In fact, we almost got one with a sapphire embedded in the band right where that circular thing is, even though that looks like a sticker or decal. Attach an antenna and you're done.
i remember seeing one of these online years ago, i always wanted one as well but never saw them released. I could have sworn at one point i saw them on ebay.co.uk but that could have been wishfull thinking.
That little tiny circle on the side is the Hitchhiker logo (the thumb). It's inset in the ring and the graphic itself is most likely some type of decal pressed into in. I don't the unit being too difficult to recreate, but that inset circle is a killer.