I live in a second story condo and my cats go out on my balcony. I don't have an enclosure but my cats are adults and have never attempted to jump in the almost 3 years they've been going out there. They have a perch/cat condo out there and love to lay on it in the sun. When they first started going out there, I watched them very closely. After this long, I am confident they aren't going to jump down. Either it's too far or they're too lazy.
You're smart though, to try to enclose it. People around here have theirs enclosed for their dogs. They use some kind of plastic looking fencing that looks kind of like a chain link fence (but plastic). I think they get it at home depot. They attach from the floor and the rails.