It would be REALLY useful to be able to rent a box for just one day, even over a weekend. All the lorry hire places round here seem to charge a minimum of two days' hire over the weekend, and have short opening hours on a Sunday.
If you hired out a 3.5t and it got stopped by VOSA and impounded as it was overweight what would you do? Or even a 7.5t with 3 horses could be overweight. You would need to put some restrictions on what they could carry in them.