Generally speaking, if you need a sleeping bag on a hike, the best place for it is at the bottom of your backpack where it can provide a soft, stable, and malleable base on which to place everything else. The sleeping bag is not often required quickly, and attaching it to the outside of the backpack risks the sleeping bag getting wet, which can create huge problems for you when you are trying to use it. While you can certainly attach a sleeping bag to the outside of packs such as the Bora AR50, it is not particularly advised. If you find your backpack isn’t large enough to accommodate all of your equipment, consider getting a larger backpack, purchasing more compressible equipment, limiting your equipment, or attaching waterproof items to the outside of the backpack.