I've got the base of the new Haul Master ATV trailer together today.
The current plan is to make
it 3 inches wider on each side by attaching wood which will also be
used to fabricate the walls.
These modifications should
allow for 9 five gallon buckets to fit snugly inside and maybe by
extending the front and back I could squeeze the total to 12.
Thin and flat metal plates aren't very stiff by themselves. A box-structure like this trailer gets a lot of its stiffness from its shape.
If you are going to extend the sides, it might be easier and will probably be more solid to just bolt the wheels &c to a single plywood base.