I guess I am assuming a lot here. if your tunnel box has a door you can't grab and has to lifted by a handle does the handle have a hinge? if so then you you can drill a hole and pin the handle so it can't function when pressure is applied. secondly if your handle design is a straight pull you can add a lockable bolt to it so it can't be lifted. I used the style that's about 1/2" thick. that way nothing is going to bend it or chew it loose. if the door is mounted so when its down into the slot its not visible. then you can just have the bolt be a blocker out side the run to prevent to the door being pushed up. I suggest concrete patio blocks 8" x 16" as pavement in the run in front of the door if you have nothing to prevent digging. that way if its just door to dirt a critter has to get past the concrete to dig under.