So with our excessive number of raccoons and possums around we need to predator proof the pen. Could we just cover the top of the pen with chicken wire?
I don't think chicken wire is strong enough. What you want is welded wire mesh. About 1/2 inch wide. Here's a picture of the cage we just built for my new pen
You must be diligent on the sides, top, and bottom. Predators and tortoises are diggers.
I have designed my outdoor pens by trenching about 8" to a foot around the pen border and then setting a treated 2" x 12" board into the top 2 inches of the poured concrete. Then using 1" x 6" treated deck boards I build a top framing that is covered with the 1/2" hard wire grid. You can design the deck boards to be hinged for access or put in a sliding system much like a double hung window. For even larger enclosures consider building a walk in enclosure about 7 feet tall.