Yup, I hear your pain about the roads. We used to regularly haul 2-3 tons in a 3/4 ton truck and I've seen nearly 20 tons of gravel in a single axle, maybe 5 ton truck. Can't get away with that here any more, the laws won't take it and neither will the trucks. 10 wheelers with an aluminum box are only legal with about 17 tons and our roads are still crap. They blame it on the weather here--many more freeze/thaw cycles than we used to get???? Maybe but more likely misappropriation of funds that should have gone to infrastructure ended up spent poorly elsewhere. Best not to elaborate more or it will turn into a rant.
As for your sealing problem, it depends on how waterproof is really needed. These boxes that dump over the lip don't leak but they have to be hinged up high unless what you haul is very fluid. Good luck with it anyway.