Shoulder Adaption

I have tried to figure out exercises to train shoulders for carrying a framless pack. There's really nothing that strains, there's no exercise that strains your neck and trepesius in the same way that hanging a back pack off your shoulders does. So i've kind of drawn a hard boundary that i'm not going to carry more than 12 or 15 pounds in a frameless packet. I split a lot of firewood, so i have abnormally developed trap muscles, um, and it doesn't help. It does dos give me extra meat on my shoulders to get sore.