Thanks for the comments.
They boxes were made for me by a local joiner who a friend knows. We found a box for sale that came in a flat pack and he used that as a template. They're made from thick mdf and are routed, glued and pinned together. He made well over a hundred of them which a few of us have used in a similar way.
Getting them level was quite a job though as the floor is not true so there's a bit of packing underneath the bottom row.