Gallon of water
A gallon of liquid weighs about 8 pounds. Both a gallon and half gallon container have nice handles that can be used to grip for increased weight. The handle shape also can be used like a kettle bell. I recommend filling the jug with water once the milk is gone and then this could be filled less or more with water to adjust to the amount of weight you’re looking to use.
Try a squat with a gallon of water!