Cliff is making a good point about secure mounting. My bracket to hold my box is 1/8" steel, the screws holding my box to the bracket are all stainless steel, and the top of the box is always locked when I am not in it.
I don't want to see anything bounce out while I am moving, and I don't want to see the whole box come free if I happen to tip the tractor. . . if that box came lose in a spill, it would make a nasty mark on my otherwise beautiful face and the wife would be really upset with me.
No matter what type of box you decide to use, make sure it stays stuck on your tractor and make sure it stays closed.