Carrying Rocks: recommendation for a durable backpack

I have a weighted vest that has about 40  pounds of weight in it, but I carry all the load on my shoulders, which is painful. I'm thinking that a backpack loaded with rocks would be better because I could carry the weight on my hips. I'm just walking around the block in it, so the theoretically better load distribution of the vest isn't an issue.

Can you recommend a backpack that might serve this purpose? I'd like to carry about 40 pounds of rock, so the pack doesn't need to be large, but it should be tough. I'm guessing an internal frame is best.