Haha, yeah I am a rockhound and was also previously in the military so I am familiar with those backpacks as well (and really not a fan to tell the truth, military gear sucks ass). I have been known to actually haul rocks for funsies in my backpack. This will destroy both military and hiking backpacks xD

I still think used weights or just jugs of water might be a bit easier in the end to throw in there. A gallon of water weighs about 8.35 pounds and a 2 liter soda bottle is about 4.4 pounds. You will have a bit of extra challenge the larger your container is because water is a liquid and therefore not static. If you don't want any sloshing, freeze it, or else just enjoy the bonus training.

You deffo want an external frame when putting weird-shaped hard objects in your backpack and to pad any corners. Even if you go with milk jugs and whatnot, I'd toss in a blanket or an old coat or something as padding or at least some cardboard.