Originally Posted by DuncanK1
Has anyone got a cunning trick to make the box more watertight or should I just resort to resealable plastic containers for my tools?
Having just finished pulling the rear bumper, "trunk", black and gray tanks from the back of our '74, I would suggest the same as the others have. If anything, make sure that you have adequate drainage holes and that there is opportunity for these areas to vent either while stationary or while you're towing. And, don't keep anything on the floor surface of the trunk that will rust or wick water.
It's unlikely in the extreme that you will be able to prevent all water from coming in, but it is easy to make things "tight" enough to prevent the water from escaping or evaporating and create a moldy mess. The combination of water and road dust makes a really muddy soup and the easier you make it for yourself to pull everything out and hose things down in there, the better.