I like X-in-6 here because changing X results in a linear change in the chance of success, and it’s simple. You make 4 traps, so the chance is 4-in-6.Instead of "x in 6" chances for random events, I prefer "1 in x".
You also get a range of different probabilities, but it's easier to remember what you're actually rolling. With a rule of "something always happens on 1", you can also roll in the open or let the players roll, and everyone knows immediately what the result will mean without having to explain it.
Another idea might be to use it as a penalty to a luck saving throw. That would roughly simulate that higher HD creatures are better at avoiding danger, and it buffs traps a bit.