I haven't built a hoop coop, but I did use cattle panels to build a garden arch for grapes. I only used T-posts to secure it to the ground so there are no cross supports. It would be plenty strong enough to hold the weight, but it does sway easily. I think you'd have to build some kind of support wall if you don't want it to move a lot as they climb.
As far as a cover I don't have any suggestions. Plywood and sheet metal would both be heavy and expensive. I'm not sure the cattle panels would make a good structure to hold them.
Maybe you could put shelves in there to encourage them to jump up rather than try to climb. A couple years ago I took in two semi-feral mom cats and their 8 kittens. An awesome neighbor lent me a wire cage, like a mini dog kennel type thing. I added shelves but didn't want to add anything permanent since it wasn't my cage. I used some spare wood panel pieces I had. They were wide but thin, so they are light weight. I drilled holes along the long sides. I used zip ties to attach the boards to the wire panel. I used wire to hold the other side level and used washers to hold the wire on the other end. The selves were sturdy enough to hold the moms and all the kittens loved using them. I've attached a couple of pictures so you can see what a mean with the wire and washers.
