The tonneau isn't a necessity, tho its not a bad idea for theft/stealth reasons. The seal between the box and the cab should presumably be watertight... Some people use an accordion style strip between the two, but using a metal bracket and any sort of strip seal will work and is relatively cheap. As far as the box itself being weatherproof, the subs and the port(s) if you're going that route should be aimed inside the cab, and whatever is exposed in the bed should be either stained with an outdoor wood stain or my preference would be for bedliner.