There are so many of these types of boxes out there, that your head will spin. I would mostly look at what the big trucks have in your area. There is doubtless a local supplier that carries them. If they don't there is always Northern. I have four Protec boxes on my truck, and they are pretty durable, unless a moron "bumps" a trailer with one when the door is open. They are pricey though. I think they priced out at about $400 a box for mine. I DID NOT PAY THAT, they came with the truck.
We mounted a storage box on the chassis under the van body of our White truck. It is 8 feet long and a couple feet deep. It is made of fiberglass and has drawers on the left side for small parts, etc. I have looked for other fiberglass boxes but none of the suppliers in this area have or can get them.