Anyone have any suggestions on a cheap, fairly dense material that could be used to add weight to something like 30-80 lbs (increasing as I get used to it, naturally) to a backpack? I was thinking 10 lb bags of rice could work, and I could probably get 5-8 in a backpack, depending on size, but i'd have to be very careful in bagging it to make sure it doesn't rip and spill. Anyone have any other suggestions for something that would be fairly cheap?