I'd say you are presented with two possiblities.
1st would be to set a large cage trap right on the toilet and move the toilet right into the cage trap. Visual curiosity lures in conjunction with this might help.
2nd option is to realize there are otter frequenting the area but they don't spend all their time at the toilet. Expand your search a little and look for channels where a swim through trap might be placed. Even a small stream might be the ticket with a little fencing.