Spurcycle started out as a Kickstarter, but is now available for purchase directly. It is very svelte and compact, and puts out a fairly good ring for its diminutive size. Its all metal, with no plastic parts.
I do think it is $50 nice, though. If your tandem is admirable (as well it should be), festooning its handlebar with a junky clunky bell makes it less so. This bell is so small as to not initially be noticed, and so nice that when it eventually is noticed it tends more to adorn than clutter. Thus, your tandem has the desired quality of being better the closer one looks, and the more expert the observer.
In doing that search, I stumbled on another Kickstarter bell, available this fall. This is more for captains than stokers, as its advantage is that it can be used while the hands are on the levers, which isn't usually relevant to the stoker.
Spec' Tarmac (road), Spec' Secteur Disc (commuter & tourer), Salsa Mamasita (MTB), CoMo Speedster (tandem), Surly Big Dummy (cargo), Airnimal (folder), a train pass, and NO car :)
My handlebars are quite cluttered on many of my bikes, and I don't have much need for a bell, but on the bikes where I have mounted a bell then I like the steerer-mounted models like this one at Amazon; fortunately ours don't have the SRAM logo on like that one does, I think I got them from a German website. That's not an option for a stoker, but it works well for a captain who only has a very occasional need.