Thanks for the input! I do the deep litter method, so there is always a fresh, springy floor for them to jump down on. Like Centrarchid said, it's more of a secondary defense plan. I've read many stories on here about people with super coops and a determined coon still finds a way in, even chewing through the wall. I've seen possums around at night, and I have traps set all around my coop, but haven't caught one in a while.
My walls are solid wood with roosts spanning the width of the coop, so there is nothing they could grab on to to climb straight up, unless they can sink their claws into plywood enough to climb a wooden wall. I had a ladder for them to get up to the first roost, about 3.5 feet high, but I watched them for a while and realized they don't even use it, they just jump up to the first roost, so I took the ladder out.