If your only interest is in keeping them from flying out, I suggest going to Lowe's, Home Depot,
Tractor Supply, or similar places and look at the wire. Choose any of the wire that makes you and your pocketbook happy. I'd avoid plastic because it goes brittle, though if it is out of direct sunlight and weather it will last a lot longer than being exposed. With those openings in the walls, it is probably not worth trying to get wire that a raccoon can't rip apart. One will just go through the bars unless those bars are closer together than I think they are.
Eleven feet is a long span. I'd get a board long enough to span it and put it across the center. That's for appearance since it looks like it is plenty high enough for you to walk under. I think appearance is important to you. How big a board depends on your climate. The more ice and snow you might have to support, the bigger it would have to be. It will be stronger if you stand it on end instead of laying it flat. Consider the strength of the wire too if you get a lot of ice and snow.
You are not going to get wire wide enough to span 11 feet. You can splice it together with J-clips, hog rings, or using wire.
There are plenty of different ways to attach it. If you get a fine enough mesh, you can use screws and fender washers, but that may not be practical unless you get smaller mesh wire. You can use poultry staples on top of the walls. If appearance is important to you, you can rip or get maybe 1/2" thick planks and put those on top of the walls. Drill pilot holes so you don't split those boards and screw those board in place. Make sure the screws go through openings in the wire.
I mentioned ice and snow load because of your comment on letting sunlight through. If you are just talking about sunlight through windows, which I think you are, ignore all that and maybe use a 2 x 6. Even if laid flat, it should not sag too much, but if you want it to not sag at all, stand it on end. For 11 feet, a 2x4 on end probably won't sag as long as it is not carrying any load.
