I've had a Yakima Rocket Box for > 10 years which is still in excellent shape and gets used quite a bit. Its plently long, but sure wouldn't fit a 6-foot ladder. The Rocket Box, though, is somewhat narrow to start with.. and the front part narrows down for aerodynamics, and would be too narrow for a ladder. A lot of boxes narrow down at the front.
But - on a Legacy I have 48-inch bars which leaves enough room for the box and 2 bikes (though its tight). So you could put junk in the box and just strap the ladder to the rack. Or do you want the ladder in the box to prevent theft? If its only the ladder going in the box, and only for theft reasons .. you could afford to replace a lot of ladders for the price of a good box !!
ron