It is not so much protection from the big girls, as getting out of sight, or slowing the big girls down. I call it bowing to the Queen. The lower birds, need to be able to show deference to the big girls. They need to be able to move away and out of sight of the dominant bird.
I have seen numerous runs, that are just an open rectangle. These runs only use the floor of the run, and there is no place in the run, where as a bird can move to be out of sight of another birds. A lot of people ignore the third dimension of space, the vertical space. This can cause problems. When a bird does not leave, does not move away with enough distance (because they can't due to the fence) the dominant bird takes offense to this behavior and pursues the bird, sometimes relentlessly.
So by cluttering up the run, birds can get behind a mini wall, 'bow to the Queen'. Put some feed bowls so that while eating at one, a bird cannot see who is eating at another bowl. A platform, allows birds to get up on top or down underneath, actually multiplying the space. It is not a separation, but a break in the line of sight.
Another trick, is to put the old hens in the lock up you are using for the chicks. This allows the younger birds to explore the area without being pursued in fear of their lives. They figure out the system, the hideouts, the roosts. Being in a panic mode, they tend to pile up in a corner, and not get away. Allowing them to look and see options can help. You chase them a bit, but not as relentless as a chicken. They will figure out how to use the set up, much better.
Sometimes it is just one hen, that pushes it relentlessly, try adding a few of the older hens, then later, a few more of them.
Good luck,
Mrs K