I have done quite a bit of digging on these little critters and the one I keep in a 50 right now is an undulated trigger which I think would be the best option, but now you've given me enough of an excuse to do more digging.
The reason undulates (balistapus undulatus) would be the best choice is because of their swimming habits. They prefer a cave structure for homemaking and don't stray far from it except to chase food. Most of the other triggers prefer lots of swimming space in addition to their large size.
I could envision a 30 with a corner cave making a good home, without excessive feeding they will grow slowly and I'm sure you have a good enough eye to see if any stress was occuring.
Plus the undulated is strikingly beatiful, not to mention fierce! Cleaning the front glass on my undulate tank has become my new "extreme" sport!
Chris