Foxman though many a otter has been caught in a castor set it is a very poor and low percentage otter getter.  Bait is also a low percentage deal for otter.  They are a fish killing machine and have no problem getting all they need to eat.
Your best bet is some sort of blind set.  Wether it be a crossover trail, a trail going to a toilet, or a narrow spot along their line of travel.