I apparently had a whole family of opossums to feed, and they were a lot more resourceful than I had previously given the species credit for being. Where the roof met is one place that was hard to seal. Any other little crack they can find. I used 2" welded wire for the outer part around a 2x3 frame, plywood nest box, and 1/2" hardware cloth for sealing up nooks and around the doors. At the bottom, the chickens would scratch and pluck the grass away eventually digging away enough for a small one to slip under. -Good luck.