Find where the entrance/exit is to the hut. It should be underwater, and you should see a shallow channel in the pond bottom. Set the 330 as close to the entrance as you can without having the trap hit the entrance when it snaps shut.
You are correct, they can become VERY smart once they escape a trap.
Have you looked into snares? They work in areas where 330's and legholds won't.
Call these guys and talk to them. They are who I use.
http://www.snareshop.com/
