I've got a Brute Force Sandbag setup. It comes with 2 tubes that you put playground sand in, one large and one small. Both of those can be placed in the main outer bag which has various handles for exercises. You can customize the weight fairly well. When I put it in my Reckoning, it stays put in a nice vertical line in the center of the pack.
-David
I have this same setup in my Seek Outside Peregrine. It's 80 lbs loaded and about all I can handle. On the bright side, the pack feels like it could handle way more weight.