A cart, if I was to do anything.
Often, I use the ballast box to trim off some dirt mounds (like a blade). Casters would just be in the way.
Also, other times, the ballast box drags when through a ditch or at the bottom of a steep grade. Casters would be in the way there as well.
With the iMatch, the ballast box picks up and drops easily and quickly, so moving it about without casters has never been a problem for me. Setting it on a dolly would also be just as easy to do, so no hook needed in the top.
I have stone in mine (thought I might like to remove it for some reason) and leave about 4" from the top to carry chain and odds n end tools. If I get a chance, will pour concrete in the box up to about 4" from the top. And will likely put the plastic tubes in the four corners for flexibility.
(and like kennyd said

)