Bakerys have those buckets too. They get frosting and fillings in them. I have done that with sand and gravel. Line the box with a tarp, fill it with wall to wall buckets and off to the quarry. shovel or rake off the top, start pulling buckets out and finish by pulling the tarp out.

Don't lose your head to save a minute,
You need your head, your brains are in it.
Burma-Shave (1947)