If you had posted your idea of filling the box with cement before you had done it, I would have suggested to you to use a 55 gallon 6 mil black plastic trash bag, and place it into the weight box prior to pouring the cement into the box. The other thing that can be done prior to pouring concrete (cement) into the box, is to wipe down the walls of the box with a heavy oil, such as chain bar oil, or gear lube. It will act as a release agent. You could have added a lifting hook, and been able to remove the concrete block and set it aside while the box was being used for another task. Concrete, or cement as it is more commonly known, contains lime, and the acidity of the lime will eventually start to cause the steel box to rust out at the bottom. Other than that, I do like your idea, and it looks like it will serve your purposes well.