working a newer job that requires a larger set of tools most of the time (as opposed to a steel foundry that requires a smaller set, and more brute force than finess) means that we need a larger tool set at all times.
the shop wanted everyone to be uniform, so they buy everyone the same boxes, enough room for quite a few things, but you tend to overflow with extra, like, where to put spray bottles of cleaners, rattle cans, rags, etc.
so i added on a side box i made with a few spare minutes in the night out of some scrap sheetmetal. a good place to put chemicals, gloves, rags, etc.
as for this vs a tool cart? we use both, some guys will load up a tool cart for the area they are in, instead of taking every single thing they have with them. others like to take their whole set with them because they don't like to unload and load their box every day. they just buy the simple plastic tool carts from northern tool and put better wheels on them.
^ before i added on the side box, it was getting to be a pain to store rags inside, and hang bottles on my handle, etc.
the after, its not perfect, but i didn't get to spend more than an hour on it or so, and i'm not complaining because it works perfectly for what it is. these tool boxes see some miles on them, and they recommended that we reinforce the box because simply using the casters it came with + no frame means that your wheels would soon be embedded with all kinds of metal bits from the floor, and over time a loaded tool box would begin to break welds (years later, and daily moving over uneven concrete sometimes).
so i made a frame out of 1/4"x2" angle iron frame, and 1/2"x4" flat stock supports in the center with a nice foot stop. it sits inside the frame evenly and flush so i shouldn't ever have an issue with weight tearing the welds apart on the box over time as it moves on uneven surfaces.
i'd take a pic of th etool carts, but they are pretty simple 2 level carts with tiny bins on the handle side, any cart would be equal