It might help if we knew which predators were causing you the problem and how you lost your chickens. What did your old fence look like? How pretty does your fence need to be? For some people in some circumstances, pretty is a very important part of the criteria. There are different techniques for different problems.
For non-flying predators, an electric fence is probably your best solution if you can provide the electricity. You don't have to cover the run that way, and with a run that size, covering it is going to be a bit of a challenge and probably quite expensive. Any top you put on it will sag so you will need some intermediate supports. If you live where you can get ice or snow, you may have real problems supporting the snow load, even with a wire top. A wet snow or freezing rain will stick and weigh it down. Whether or not you cover it will make a huge difference in what you build.
I don't know why you have limited it to a "frame" fence and I'm not exactly sure what you mean by that. Are you planning on using any wire or is it totally made out of wood? I don't know enough about what you want to do to offer any specific recommendations. I will throw out a few comments that might or might not help.
I use the philosophy of locking them in a predator-proof coop at night and keeping them in a predator resistant run during the day. I used to free range but had too many problems with people dropping dogs off in the country so the people out here can shoot them. They can cause problems before we shoot them though. Since I’ve adopted the predator resistant run (actually electric netting) I have not had any losses.
If you do not put a top on it and you put a top rail on the fence, chickens may fly to the top rail just to perch. They enjoy perching. When they hop down, there is no telling which side they will hop down on. And once they get out, they don’t know enough to get back in. If the top of the fence is wire so they don’t see a good place to perch, they generally won’t fly out.
However, if they are motivated, they have no problem flying out of a 5 foot high fence. I had some hens learn to leave the run when I was using a 5’ high fence. They were trying to get away from an amorous rooster. If properly motivated, they can get out of a very high fence. With a big run like you are talking about, you should be OK. A risk is if you integrate chickens, they may be motivated by trying to get away from the bullies. I occasionally have that with my 4’ high electric netting.
I personally don’t like putting any more wood touching the ground than I have to, even treated wood. It will eventually rot. That is just personal preference.
I have seen animals like possums and groundhogs just run under a fence if there is a tiny opening between the ground and the bottom of the fence. You’d be amazed at how little room they need. I'm sure raccoons, foxes, and coyotes can do the same. I like an apron around the run to stop that as well as stopping digging predators. Take a section of wire about 18” to 24” wide, lay it horizontal on the ground and attach it to the bottom of your fence. It stops them from just pushing under the fence and if one decides to dig, they hit the wire and don’t know to back up. You don’t really have to bury it at all but it makes it easier to weed eat if you take the sod off, put down the fencing, then put the sod back.
I don’t know of any of this will help. Good luck!
