Just like any tool box- if it is sealed its ok. Having that sealed well enough for places that everything rusts in-that would be hard. I wouldn’t think that would last. Here you would not even bother having it sealed up. Haha.
Might as well get a decent utility box with weatherproof and locking side compartments for the money put into fabricating that. Makes you wonder, too, what happens if his rear slides out on him and the tire hits a curb, does everything behind that bed side pop out and spill all over? What of a minor fender bender into a rear quarter of the bed? Or driving 50 down a washboard gravel road, would the outer skin pop open like a gullwing and everything inside go bouncing outside down the road?