All the guitars I have seen have been lashed or bungee'd to very large Osprey or external frame packs, typically belonging to hitchhikers along Highway 1. smile

The more ingenious way was hooking a bungee in the large opening on the face of the guitar, running it through lash points on the pack, and bringing it around to hook on the other side of the hole. Not that this is likely to be good for the guitar. Another bungee lashed the neck higher on the pack.
"In the beginner's mind there are many possibilities. In the expert's mind there are few." Shunryu Suzuki