Another way which I did is to use wood towel hangers that you can get at Home Depot - mount the ends to the ceiling and use a long dowel instead of the towel rod. Hang it above your bi fold doors. Hang a piece of batting as wide and as long as you need it, using curtain hooks. This way if you need to get it out of the way, you can push it to one side or the other.
I find that batting works better than flannel.