If your run is Fort Knox and your confident that predators cannot get in over the top, through the sides, or by burrowing under... a lean-to style shelter may do fine for your situation. I would be most worried about prevailing winds and rain - particularly if you get the dregs during hurricane season. Wet and drafty are a bad combination in regards to flock health. Be aware of the weather on your property and take that into account when deciding the location and direction of your shelter. Make sure their run provides enough shade in summer and that their feed is protected from getting wet.