I don't see a photo of the type of container you are using. You need no more than 35 psi (pounds per square inch). For a top surface of 3 x 3 inches, which would be about 9 square inches, the total weight applied would need to be 9 x 35 = 325 pounds. (The arithmetic is the same when using other units of measure.) You can measure the applied weight roughly, by tightening the clamp directly onto a bathroom scale, in order to get a sense of how much to tighten it. If the top surface of the container is much larger than a 3 inch square, then you may need a sturdier container. Also, be aware of any ridge extending below the bottom of the container, since this would need to survive all the weight.
Bob