i know everyone is going to say dont try and put one in there not enough room but if you buy a small bursa i think you will be ok i have a huma and a niger both babys in a 40 gal so you could get one of those but you have to know what you are going to do when they out grow the tank which they will you can try any one except for the undulated titan of course and last i think the clown because it is very expensive and in those small tank circumstances it would be a waste of a lot of money and a beautiful fish.
I would go with a bursa they dont grow as big as the rest
You can keep a trigger in there for a couple of years. I would go with the less aggressive triggerfish. These triggers are male/female blue jaw triggers, sargassum triggerfish, and a pink tail. These are the most non aggressive triggers. Good Luck! Also, purchase a smaller size triggerfish so it isn't as mean. Good Luck!