I've never made a wall hanging that big so I am just brainstorming with you. My suggestion would be to make a sleeve at the top and bottom of the wallhanging and insert dowel rods. If you secure both top and bottom to the wall, it might prevent it from sagging. I'm thinking this is going to be a wall hanging with significant weight to it due to using the ties, stabilizer behind the ties and then fusing them to your back ground fabric. Love your tentative lay out though!