This particular brand of box had ribs down the sides for added strength. They also fit a sheet of thin Masonite perfectly so I was able to slide these dividers into them. As an added bonus these particular boxes are clear so I can see into them without opening the lid.
Shown here is my car camping "Chuck Box" made up mostly of my backpacking kitchen and some extra luxuries that are a fringe benefit of camping next to your vehicle. I've made dividers for two more such boxes - one for bike tools, helmets and Camelbaks and another to carry my posh camp amenities like air matresseses, lanterns and such.
I have no idea where I bought these (maybe I "borrowed" them from work) - the brand is KeepBox. They are available from Office Depot but they are a bit pricey so you may want to shop around.