Old steel roof paneling.
1/2" hardware cloth.
1"x2" fencing offset to make the holes smaller.
Lots of options here.
If that was mine, I'd cover the entire thing with 1/2" hardware cloth and put a tarp over it or a metal roof on it, if it could handle the weight. Also add a 3 ft 1/2" hardware cloth apron. I have all the predators. Secure the openings around the doors to be no more than 1/2" at each gap. I secure 2"x4" to my runs to make the door gaps smaller wherever the structure can handle the weight. There are other smaller sizes of wood also that will close up those gaps. Metal zip ties are a good option for connecting fencing together and to the support poles. Hog rings are ok but can be a pain to install on 1/2" hardware cloth. Fender washers with screws are useful when securing hardware cloth to wood, or sandwich the hardware cloth between two pieces of wood.
At a minimum, run about 2 ft high 1/2" hardware cloth along the base of the entire structure, give it a 2 ft+ apron, and if there are roosts that stick through the sides, put about 2 ft x 2ft area of 1/2" hardware cloth where the roost touches the sides of the structure. That way if birds fall asleep on the ground or on the perch, a racoon can't reach in and tear their head off. Think about how long racoon arms can reach, and that's the amount of area to protect. The 2 ft high hardware cloth will also help some with snakes and other predators.
