historically i have followed james duncan davidson's article "Sandboxing Rails with Darwin Ports" : <http://duncandavidson.com/essay/2006/04/portsandbox>
james also offers some commentary on installing via package management vs. by-hand.
the topic in general has been discussed a gazallion times on the (old) list before, it might be wise to search the archives and see what you can scare up.
regards, jean-pierre