I played with the design a little and got it pretty dang good "I think" with a 1.5" opening that has the flap doors with barbs.That will open to 4" if bigger ones need to push in.
I got about 75% done with 2 cylinder traps today. Spent some time coming up with a bait container that is suspended in the middle so the little thieves can't eat without going in. Like shrimper was talking about.