Use them as storage bins for small scrap steel pieces. I would talk to your yard and see if that's acceptable. You could do exceptionally well, if you have the storage space as you described. I would use it like a savings plan; fill as many as you can and wait for prices to peak, sell all at once. Just my opinion. I don't see a yard having an issue, the ratio of fluff would be rather small if you filled the carcass with good material, especially in bulk.