What you described is exactly what I use mine for. It's nice when you're on the road knowing you can lock it and have some level of comfort that it's secure (and not all jammed in behind your seat).
Some advice:
Before you install it, put a good coat of wax on the sides and front since you most likely won't be able to keep them as clean as the back and top. I suggest waxing the back and top when you wax the truck to keep the box shiny. Mine is 9 years old and I buffed it this past summer for the first time and it still looks like new. Also check the latching mechanism when it's shut. Mine latched and locked but it didn't seal tight. First trip I made in a cold/powder snow storm, the snow swirled around and somehow got sucked up into the box. I adjusted the latches so it closed tighter into the seals and I haven't had a problem since.